I put a new digital heat pump themostat in and I set it for 73 degrees I day it would run and hold the temperature but the last 3 days in the afternoon won't run and I have to keep turning it to cool and off and then finally it will start to ran. What would be the problem?

I put a new heat pump themostat in and I set it for 73 degrees in the early day it will run but around afternoon and won't run and I have to keep turing it to cool and on and off and then it will start to run why?

Must have been a Hunter tstat