Faulty TRV

I have Myson 2 way TRVs on my radiators but there are 2 radiators that are not heating up.

All radiators have been bled and were previously balanced.

When I remove the plastic body of the trv from the radiator, the radiator will get piping hot very quickly so that side of things appears to be working correctly.

I compared the trv body with that of a working one and the pin that controls the flow rate does move but is very stiff in comparison.

Is there any way to adjust this or is it just faulty??

Try a squirt of DS40 ?

I don't think its stiff due to friction, more to do with internal workings I reckon.

On good TRV the pin was actually loose when on hottest setting.

On faulty TRV the stiffness of pin does change with temperature adjustment but not in the same range