floproblem

I was cleaning a boiler for a coustmer,they asked me to look at a radiator that wasnt heating.Its a monoflo system. The main pipe runs toward the east side of the house the splits to feed the north and south sides, after the split there is a tee on the north side that feeds a pipe to a room and the return from the room goes to a monoflo tee on the south side supply pipe. could the fact that the radiator is connected to two differntfeed pipes be causing the flow problem. Or should i look elsewere.
Thanks
Rich Traupmann