During the winter months, it is normal to lose a few degrees when using your spa. The water heater can’t compensate for the heat loss quickly enough. Depending on the number of motors in the spa, when they are all activated at the same time, there is not enough amperage available to keep the heater running. In addition, the blower takes in cool air from under the skirt, which also has the effect of cooling the spa water, as this air is not necessarily warm.
It's also important to make sure that the spa is not in Economy mode, as this mode will maintain a temperature 20 degrees lower than required. You can find more information on this in your customer access. You will need to select the keypad model that is incorporated into your spa.