Check your jet intakes for any dirt or debris.
Remove any dirt using a garden hose.
Fill your hot tub with water and set the temperature to your liking.
Turn on your hot tub.
Check the temperature of your water after 10 minutes.
Wait for the temperature you set to be reached. Your hot tub automatically will work on slow cycle until the temperature you set has been reached.
Wait an hour and check to see how much the temperature has increased. If it has barely moved, you might have dirty filters, which is why the hot tub won't pick up to a higher speed.
Turn off your hot tub.
Flush the hot tub filter with water to remove any dirt.
Turn on your hot tub and adjust the settings to the low speed.