Hot tub heating

Hi, we recently bought a house and it has an indoor hot tub. We cleaned it, filled it and added the soft soak package from you guys. It works fine for about 12 hours and then it stops heating. The light on the panel says the heat is on but the temp keeps dropping slowly. If I turn the breaker off and wait several minutes I turn it back on and it starts to heat up again to set temp. After about 12 hours of fine operation it stops heating again and temp starts dropping. The pump continues to run and the heat light is on but tub slowly cools off. I added a pic of my heater to give you an idea of what equipment I have