TRV plunger not retracting enough after battery change

[sberry]

When installed from new, my radiator TRV worked fine.  However, when it came to changing the batteries, the TRV wouldn't open enough to allow any heat into the radiator.  I replaced the batteries and calibrated the TRV, but I noticed that, when inserting the new batteries, the plunger goes all the way into the body of Wiser head, however when it asks for heat, the plunger only retracts 1mm or so.  To get any heat into the radiator I have to back off the head by 3/4 to 1 turn.

More and more of my TRVs are exhibiting this behaviour.

Any ideas as to why this is happening and how I can resolve it?