In my home I have a few Evohome HR92 valves and I'm really satisfied with the system, apart from a problem with the one in my living room.
Before replacing anything I'd like to make sure where the problem is.

The valve works fine most of the time, but sometimes it stops working and doesn't open any more when it's getting cold. Increasing/decreasing the temperature makes the valve make the normal rotating sound, but the valve doesn't actually rotate so I'm not sure what's happening.

The thread on the valve is in perfect conditions so I don't see how the motor can rotate without doing anything to the valve.
The adaptor valve (if I remove the motor body) is very easy to twist with my hands and works fine.

The weird thing is that, if I remove the valve and put it back immediately without doing anything else, then it just starts working fine and goes on working for a few days.

Any idea what he problem could be?