Spa pump wont run when in spa
A few possibilities - 


The pump is frozen but the motor works - The motor cannot turn the
pump.  With the motor off of the pump, the pump is difficult to turn. - Fix the pump
3 phase motor driven with just 2 of the legs - something is disconnected somewhere - I've had this issues with a loose/melted wire nut on a motor with large pump that had this same issue. - Fix wiring
The motor works without load, but is not able to work with load.  Testing is tricky - try other possibilities first - problem could be motor, but more likely the driving circuits.