Radiators not heating?

Hi
Please can someone advise on the following?

We have something like 32 radiators in our house and 3 of them are not heating up. Its a wet system and somewhat old in places.

I have tried bleeding them, checked the TRV valves are moving (which they are) and have checked that the lock shield valves are not closed.

There is heat on the in pipe, but no heat whatsoever in the actual radiator or out pipe.

This is the first Autumn we have had in this house so I don't know if this has worked before, but I presume so as the previous owners would surely have noticed!

I'm really not sure what to try now. Any thoughts would be greatly appreciated and thanks to everyone that posted about the TRV's as that helped me tremendously so far.

Sue