Toilets, Sinks, Showers, Tubs and Disposals - Temptrol might need service

I have a Symmons Temptrol valve in my tub/shower that had been working without a problem. When I reduced the temperature of my water heater, I wanted to adjust the stop limit on the valve to get a little more hot water. Unfortunately, my plan didn't work and now the water doesn't get more than very warm. The water from the sink is still quite hot so I know it isn't the temp adjustment at the water heater (also I didn't adjust the stop limit for a couple of days after I adjusted the water heater and the shower/tub was still pretty hot). My thought is that when I adjusted the stop limit, some gunk got shook loose and is impeding the proper movement of whatever controls the flow of hot water. Is that possible? Is there an idiot proof way to fix it? Thanks.

I removed the flow control spindle to inspect it and the piston 'seems' to move freely. Is there something in the stop spindle assembly that could need checking? I think I need a special tool for that one so I won't be messing with it yet.