This can be caused by an air lock.
Remedy: Open the cabinet door and slightly loosen the barrel union to the pump, (apply this to whichever pump is not operating if you have a 2 or 3 pump system) this will allow the trapped air to escape.
Turn the spa on, then, when the water starts to come out of the barrel union, tighten it again. This should remove the air lock.
For further assistance, please contact your Dealer.