Tub/Shower temperature

I have a Symmons temptrol valve in our shower. The problem we have been having is that we don't get hot enough water to draw a bath- yet strangely (to me anyway) the shower is hot enough. When drawing a bath, the water is hot for about 30 seconds and then goes lukewarm (about 85 degrees). In poking around the net I came to the conculsion that the spindle/diverter was bad and so I replaced it and was very proud of myself- except it didn't fix the problem.

The sink in the same bathroom gets hot water (about 120 deg) and the washing machine located 1 floor directly below the bathroom also gets hot water with no problem. What gives?

We have been filling pots of hot water from the kitchen to supplement the kids bathwater... so I would really like to figure this out!

Thanks for any advice and thanks for the forum...

I did just find this post also:
https://terrylove.com/forums/showthread.php?t=24915&highlight=symmons

...if anyone would like to suggest/add anything else- please do... thanks!

wow- same problem with a Symmons... obviously this has happened before..
https://terrylove.com/forums/showthread.php?t=6202&highlight=symmons

The pressure balancing is getting stuck. I have found these valves are very tempermental when it comes to foreign materials in them.