Bluefish: My system is over ten years old, and is really two systems interconnected (i.e. different diameter pipes). I’d bled the rads and also balanced them last year and everything was OK, but this year the rads furthest away from the boiler on the ground floor kept getting colder and then those the farthest away did not heat up. The contractor arrives and says that nobody came to my house when I was down in the pub and unbalanced the rads, and the problem was most likely the pump was not powerful enough. Possibly a blockage in the small diameter pipes. So he replaced the pump with a nice Gundfoss one, it cost about €350 incl VAT. No problems since then.
You could try bleeding your rads to make sure there is no air in them and then balance them. This not particularly difficult and there is plenty of advice on the web on how to do this. If this doesn't work you should consider a contractor.