Bleeding Radiator!

I have put a new radiator on the wall in the living room, the old one was 36 years old and not putting out a great deal of heat. It went on like a dream, even though it is a bigger one and I had to adjust the pipework accordingly. I filled it up and bled it, put the heating on only to find that it's not heating up properly. It's fairly hot at the top but only luke warm from the middle down. I've been given advice by people and have tried different things but it's still the same. Wish I'd left the old one on.