Check the expansion tank. Make sure it isn't waterlogged or the air pressure isn't too low (note, you can't check it with the system under pressure, it will just read that rather than the static air pressure). If you had excessive air, the water probably wouldn't circulate. If the radiant stuff gets hot, you should be okay in that regard. If you don't have an expansion tank, someone messed up, add one!
If that isn't it, it could be the autofill valve, or the fill valve may be leaking.