Return pipe cold, TVR Hot

Hi everyone, ive had a problem with my living room heating for a while and cant figure out what is wrong with it to rectify the problem,
when i first start the heating up all the radiator heat up no problem including the living room rad.
after around 30 minutes the living room rad cools down even tho all the other radiators are Very hot, after feeling the TRV and the small amount of space between the tvr and the rad the space also hot, but the rad and the return valve are cold, all the other radiators keep hot but the living room one cools to the point there is no heat at all, i have a magnafilter installed on the system and this is clean, ive also balanced the radiators just in case this was the fault and anyone help me identify how to resolve this as i now have a Baby in the house and i don't want him getting cold

If the living room rad heats up every time, all be it for a short time, the flow & return pipes must be clear.
This may suggest a fault with the trv shutting down somehow.
I would change it for a regular valve just to see what happens. If I'm off the mark I'm sure someone will put you right.
(the trv would still feel hot due to conduction)

Remove the sensor head, make sure the pin is up. Job done till you can fit new valve= warm baby.

Oooo, I used to have a TVR (Tuscan), it was hot too.

+1 with @45yearsagasman, if it runs ok with the head off then just replace the head/valve