How can i fix a sticking Triton thermostatic mixer shower?

When I turn on the flow control knob on mixer shower it will only turn on so far before it becomes very difficult to turn. I can adjust the water temperature OK but it is hard to increase the flow. If I force the flow control knob past this sticky point the flow increases and becomes easier to turn but the same problem occurs the next time it is turned on.

Billc
August 2010