The system is leaking.
Do you have buried return lines? If so, they may be leaking.
Are your air vents in good condition? You can lose plenty of water through a leaky air vent.
Are all the valve stems holding tight? A bad packing gland can waste as much steam as a bad air vent.
How about the boiler? Is it in good shape? When you look into the chamber, do you see any rusty areas? Do you see any discolored areas on the burners? The boiler may be leaking.
There may also be a hole in the boiler at its water line. Raise the water level into the header and see if water pours out of the boiler. If it does, you'll have to replace the damaged sections.
The boiler water pH is too high.
The pH should be between seven and nine. A pH of 11 or higher will make the boiler water foam. Foaming water will leave the boiler with the steam. That lowers the boiler's water line and causes nuisance low-water problems.
If there's an automatic water feeder, it will add water to the system. When the condensate returns from the system, the boiler will flood. Correct the pH with chemicals. Ideally, it should range between seven (neutral) and nine (mildly alkaline).
The water backs out of the boiler.
This problem often pops up with a gravity-return (no condensate- or boiler-feed pump) system. If the boiler pressure is more than the "B" Dimension can overcome, the water will back into the return lines, and often cause water hammer at the ends of the main. The water hammer can damage the steam traps.
If the boiler has an automatic water feeder, the feeder will add water. At the end of the heating cycle, the condensate will return from the system, and since the feeder added water, the boiler will flood. Try lowering the pressure.
And keep in mind, the Dead Men designed most gravity-return, two-pipe systems to run well at extremely low pressure (typically in ounces). If you can't heat the building at low pressure, the system is probably air-bound. Check your air vents.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).