Refitting TRV head

Hi

When we moved into our current house most radiators had TRVs fitted, but some had the plastic head unit (Myson Contract 2 way) removed. I found the head units and tried to refit them, but whenever I do those radiators go ice cold, as soon as I removed the valve they are roasting again. They go gold even if I only screw them down a little.

I had a look at the wiki so if I missed anything please point me in the right direction.

Any help would be appreciated, the warmest room in the house is barely used so we would like to be able to control it a bit better.

Thanks in advance!

Turn them onto number 5 before you fit them that makes sure there fully open, then turn them down to number 3 (20deg) or number 2 (16 deg)