This happened to my friend last week and i was there when the man from British Gas came round. he said that if bleeding the rads didn't work it meant they were full of sludge (his words not mine) and they would need to be flushed out. This is quite expensive, even if you have cover with british gas (as it's not covered in the policy) so bleed them and leave it a few days and see if that helps.
You have to bleed all of them, not just the ones that are not getting hot. The mistake my friend made. After three days hers were fine.