how to correct an unbalanced heating system?

I am dealing with an heating system in a flat. The Dining room and bedroom are at the rear of the property. The radiators in these rooms feed off the bathroom. The pipe work goes up, across the ceiling and back down and the return pipe is routed next to the feed pipe.
I have replaced two single radiators with two double radiators but these two will not heat up. The single radiators were not very warm.
Is there something I can do to correct this?