My hot water line runs under my slab in a loop:

Hot water heater > Master Shower > Tub > Kitchen Sink > Guest Bathroom

The last couple weeks the master shower has only been getting luke warm. I believe its a Delta R22000 (edited from thinking it was a moentrol something) but i'm having a hard tiem finding my manual or order email. Its a 3 zone, 6 stop. 2 handle. I believe it does something with temp sensing? It pretty much HAS to be in the valve thats causing the issue since everything else is getting hot. Is there anything I can do or check?