Wiser Thermostatic Radiator Head Not Fully Opening

[DaveP]

If I twist and hold the head clockwise the red led lights up and the actuator fully retracts leaving a small gap to the TRV pin, so I know it can physically fully open it. I then twist it anticlockwise and the blue led lights up and the actuator closes the valve and after a while opens it partly i.e. calibrates it.
My bathroom is very cold and doesn't reach the set temperature as the valve hasn't fully opened; if I unscrew the head until it is almost off, allowing the pin to come out further it obviously opens the valve up and the radiator gets warmer.
I have tried this with the system hot, cold, removed and replaced the batteries, tried a different valve.
What is the reason and solution please?

[Polarismd]

Is the pin in the valve stuck? Sometimes they do. You might need to get a pair of long nose pliers on it and manually push it in and out. Some Silicon Spray (WD 40 do one) will help.