If the piston is moving smoothly but your TRV is not operating, the internals are likely kaput. It happens now and then, especially in older models, so take this opportunity to upgrade your home heating and benefit from proper TRV function. Fit a new TRV, confirm the valve works, and years of worry-free modulated heating should await you.
Avoid finding out your thermostatic valve is broken too late, as well as preventing a sticking TRV, by seasonally checking and lubricating your valve piston. It is also important to keep the vents around the valve head free of dust and other debris as these may lower the accuracy of your TRV readings.