There could be several possibilites
1- The hot water source has changed the out put temperature
2- The incoming cold water temperature could have gone up or down.
3- The sensing element may have lime or other deposits which may insulate the element from responding in a normal time.
4- Your piping may be galvanized or other ferrious type with a build up and thus again restricting the full flow of water
Rather than guessing all the possibilities why dont you just call a Licensed AND insured Master plumber to disassemble this shower body and find out first hand what is really going on BEFORE this device fails and shuts down in the fail safe mode. good Luck
SylvanLMP