The system doesn't have main vents.
Steam will always follow the path of least resistance. Since it's a gas, it doesn't understand the difference between up and down. All it knows is OUT! Steam will always head for the air vents.
If you don't have any main vents, the steam will compress the trapped air and stop moving. If you have main vents, steam will travel quickly through the mains and reach the radiators in no time at all.
Main vents belong near the ends of the mains, but never put them in a tee right at the very end of the main. If you do, water hammer might clobber them within the first few cycles. Pipe your main vents at least 15 inches back from the end of the main, and six-to-ten inches up on a nipple. This means you may have to cut and thread the pipe in place to get the main vents in, but it's well worth the effort.
Don't install a three-quarter-inch vent in a one-eighth-inch hole you drilled and tapped in the main. You can't vent much air though a hole that small.
You hear air hissing from the air vents.
In a properly vented steam system, you won't notice the air venting. Think about it. The fact that you can hear the vents hiss means the air doesn't have enough ways out. The hissing is the sound of high-velocity air trying to get out. Why is the air moving at high velocity? Because you don't have enough vents.
High-velocity air can carry sediment toward the working vents and clog them. So when you hear the vents hissing, they're trying to tell you something. Add main vents (or more main vents) to the system.
The boiler is making wet steam.
Check the near-boiler piping against the manufacturer's specifications. If the piping can't separate the water from the steam, the water will condense the steam, and it will take a long time to heat the building.
Check, too, the water's cleanliness and pH. You may have to clean the boiler and the system, and balance the water's pH with chemicals to get it right. A good pH for a steam boiler ranges between seven (neutral) and nine (mildly alkaline).
The burner isn't firing to the connected load.
The burner's ability to produce steam has to match the system's ability to condense steam. Your firing rate must match the piping and radiation load. If it doesn't, it will take a long time to heat the building. A flame that's too small is the equivalent of trying to melt an iceberg by spraying warm water on it from a garden hose. Check that flame against the connected load and adjust it, if necessary.
The steam pressure is too high.
Check the cut-in setting on the pressuretrol. High pressure can shut air vents and keep them closed. You'll be pushing steam at trapped air, and the building will stay cold because the air can't get out. If you set your pressuretrol's cut-in setting at one-half psi you'll never have a problem.
The pressuretrol's cut-out setting is another thing. It should be no higher than the pressure it takes to get steam from the boiler to the furthest radiator. The pressure you need is a function of pipe size and boiler size, and the Dead Man figured it out years before you and I were born.
When in doubt, crank the pressure down.
There's a water leg before the condensate- or boiler-feed pump receiver.
In two-pipe systems, you have steam traps on the radiators instead of air vents. The steam pushes the air through the traps, into the dry return and toward the condensate- or boiler-feed pump receiver. The receiver has an atmospheric vent, and that's where the air is heading.
If your return line drops below the inlet to the receiver, however, you'll have problems. That return line isn't under pressure because it's downstream of the radiator and end-of-main F&T traps. As condensate drains from the radiators and pipes, it will pool in that water leg and form a seal. Air won't vent through that water seal, and it will take forever to heat the building.
If the traps are defective they'll mask this problem because there will be enough pressure to force the condensate out of the water leg. A lot of water hammer usually accompanies this evacuation of the water. The noise usually encourages the building owner to have his traps fixed. But once the traps are fixed, he'll have little or no heat because the air can't through the water seal. At this point, you have two choices. Raise the return line to eliminate the water seal, or install main vents at the outlet sides of the end-of-main F&T traps.
There's too much radiation attached to the riser.
A steam riser can carry just about any load if you get the pressure high enough, and if you don't care about velocity noise. The challenge with steam heating is to deliver the right load to the radiators using low pressure, usually not more than two-psi pressure at the boiler.
The Dead Men sized the radiators to heat the space on the coldest day of the year with about one-psi pressure at the radiator. They used low pressure steam so the radiators wouldn't overheat and the fuel bills wouldn't soar. They used pipe-sizing charts that showed them the load limits for steam heating (See The Golden Rules of Hydronic Heating). If you connect too much radiation to a steam riser, you won't be able to heat it all unless you raise the pressure to an abnormally high level. And when you raise the pressure, you create other problems: high fuel bills, water level problems at the boiler, and noise.
Check the radiation against the carrying capacity of the pipe. Try closing some radiators to see if that helps. If it doesn't, you may have to repipe.
There's sludge in the horizontal runout to the riser.
This often happens after a one-pipe steam system floods. Water works its way up into the radiators, and the sludge washes down into the horizontal runout to the riser. A puddle of condensate gathers around the sludge, causing the steam to condense. The radiators won't heat properly because the steam can't make it past the sludge and the trapped condensate. If you remove a radiator and shine a light down the riser, you'll see a reflection. That's the trapped water.
Break the riser at its base and flush it from the top under pressure from a garden hose.
The riser needs to be dripped.
In a one-pipe system, all the condensate from the riser and the radiators returns through the horizontal runout to the riser. If too much condensate falls down that riser, the steam won't be able to move toward the radiators.
Drip the line if you can. If you're dripping into a dry return, use either a steam trap or a loop seal.
If you can't drip the riser, increase the size of the horizontal runout to the riser by one size over normal, and pitch it at least one inch per foot back toward the main (see The Golden Rules of Hydronic Heating for pipe sizes based on connected load).
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).