You probably have one or more air-locks in your system. You need to 'bleed' the air out - it's very simple. Buy a radiator valve key from any good hardware store, use it to gently loosen the little nut you'll find at the top of each radiator. You will hear air escaping - as soon as water appears, tighten the nut back up again - finger tight is ok. Apply to each radiator in turn, starting with the lower floor first, and finish in the bathroom. You may need to do this a couple of times, leave the system for a day in between each attempt. This should clear the problem - if not, it's a call to your plumber, but this is an easy, and free, method that usually sorts the problem.