The system is corroding.
All steam systems are open to the atmosphere. Whenever you mix water with iron and steel you're going to get rust, and that rust is going to wash down into the returns. Eventually, those returns will clog, and the condensate will rise into the steam mains. The symptoms of this problem are water hammer at mid-cycle and water level problems in the boiler. You should regularly clean and flush the returns.
The system is taking on too much feed water.
Fresh water contains lots of oxygen, and oxygen causes corrosion. Feed water also contains minerals that will settle out when you heat the water. Those minerals (mostly calcium and magnesium) form a rock-hard scale that can quickly clog your return lines.
If you have an automatic feeder and any buried steam or condensate lines, it pays to install an inexpensive water meter on the feed line. Keep a log of how much water enters the system. It's normal for all open steam systems to take on a some water, but if you see the meter reading suddenly jump, you know you have a leak. Find it and repair it.
You have to clean the clogged returns.
To do this properly, you'll have to disconnect the returns at both ends and flush them through under pressure. This may be harder to do than it sounds, especially if you don't have anywhere to put the crud you'll be flushing out of those old lines. And even if you can force water through under pressure, you may not be able to clear those old lines. Remember, once you reconnect the return lines, there will be very little pressure available to make the condensate flow back to the boiler.
If you flush returns regularly, they'll be less likely to clog.
The return doesn't have a flush valve.
Chances are you don't have a way to flush those lines without taking them apart. But if you get an opportunity to install a flush valve, it will help you clear the lines the next time around. A flush valve on a steam system is like a purge valve on a hot water heating system. You install a gate valve on the return line, just before it enters the Hartford Loop. Right before the gate valve, install a tee with a full-size, full-port ball valve.
To flush the lines, close the gate valve, and fire the boiler. When you build up some pressure, open the ball valve. The steam will push the crud out of the system. You'll have to feed the boiler while you're doing this, of course, because the condensate won't be returning from the system. This method works especially well when you're trying to clean a new boiler.
All steam systems need to be flushed regularly.
You have to replace the wet return.
If the wet return is clogged beyond salvation, you'll have to replace it. If you're dealing with a buried return line your job is going to be a tough one because you'll probably have to jackhammer the floor. If you put the return back in the same place, wrap it in foam insulation to protect it against the concrete.
You'd like to replace the wet return with a dry return.
Be careful here. There's a reason the Dead Man ran a wet return. On a gravity-return system, there may not be enough height between the end of the main and the boiler water line to allow the condensate to return through a dry return. Or there may be risers dripping down into the wet return from upper floors.
If you use a dry return, steam might have access to those drip lines. It will flow up the riser as condensate tries to fall down. That can cause water hammer. It's possible to run a dry return, but you'll have to use loop seals between the riser drip and the dry return. Go ahead and raise your return line, but connect your riser drips from their present location at the floor into your new dry return. What you'll wind up with will be loop seals. A loop seal is a U-tube that fills with water and keeps the steam from working its way up into the riser drips.
You'd like to use a condensate-transfer pump instead of a wet return.
It may make sense to abandon an old wet return and pick up the condensate with a small condensate-transfer pump. From the pump, you can transfer water back to the boiler though a much smaller line (usually three-quarter inch). You can also run this line overhead if that's more convenient. You'll need a properly sized F&T trap at the end of the main, just before the condensate-transfer pump. Drain the F&T into the transfer pump, and run your new line back to the boiler room. Ideally, you'll be dumping your condensate into a boiler-feed pump, but it's also feasible to go directly into the boiler from the transfer pump.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).