We had problems with some of our radiators too which lordalex solved by 'rebalancing' them.
Try youtube to see what to do. He did and didn't seem to find it difficult.
Almost certainly gunge due to lack of rust inhibitor in the system. If you bleed it and the water coming out is black then that is the answer. You need to flush the radiator and put some rust inhibitor in the header tank - I use Fernox 1B.
One side only? That suggests the water isn't circulating, either a stuck valve, or it needs flushing, especially if the pipework is micro or mini-bore.
Even with 15mm copper tube, it doesn't take much sludge to bung up a valve.
get right type of radiator key. insert very gently and gently turn - hold a piece of toilet roll under and when it is black = your top shoudl be lovely and and hot.; foggy typos.