Valves

I have a grohe thermostatic shower. The dial ranges from 60 – 110F. When it is at it’s hottest setting the water is only warm and never gets hot. If I turn it down, and slowly turn it up it may get hot for 2 seconds and then returns to warm. We have checked the hot water heater, and can get hot water from other faucets, so I am assuming it has something to do with the Thermostatic Valve. Is there any way to fix this myself and avoid calling the plumber? Any advice will help!
Thank you.

it sounds like the valve has a lot of deposits in it plugging the flow. you might be able to remove it and clean it out.

What is a triple handle shower system as opposed to a double handle system?