Shower waterflow control becoming very hard to move?

I have a Triton unichrome dart thermostatic mixer shower, which is not very old.
The waterflow dial has over time become more and more stiff, and it's almost at the point where I need to use a pair of pliers to actually turn the shower on! It turns off ok once I've used it, the water temp and pressure are all fine.
Any ideas what it could be?