hot water heat

I'm having trouble with my hot water heating system.I have an HBSmith boiler that has worked fine in years past.This year however it has given me  fit.I found that the recirc pump was frooze up and replaced it.I've replaced the bleeder vents on both the supply and return sides,I've bled each baseboard type register.My boiler comes on but only runs for a short while.I do have some heat at one set of registers but at none of the others.When I bled the registers all I got was water,no air.LAst year and years before when the boiler was heating you could hear the water run through the pipes as though there was air in the lines.You can't hear that now.I don't know what else to do.Any suggestions?