Which model Herpstat do you have? If you have the original Herpstat is the LED to right of the temperature readout staying consistently red? If this is the case then the power is being applied full on and your heat device is unable to reach your target temp. This could be because of the room getting cooler, the temp probe slipping off the hot spot or the heat tape going bad. Same would apply if you have one of the other models and the power is constantly reading 100%. Full voltage is being applied but the heat device isn’t putting out enough heat. To test that you could always unplug the heat device from the Herpstat and plug it directly into the wall leaving the temp probe from the Herpstat in the same place it is so it acts as a thermometer and see what it gets up to. If it still only comes up to 89 degrees you know it is not being affected by the thermostat.

George