The fact that pump runs one direction and not the other indicates that the switch has both B+ and ground. There is nothing special in the motor that makes it reverse direction. All the switch does is reverse the polarity. This swaps the magnetic pull and the motor rotates in the opposite direction then before.
I would test the voltage at the switch. In order to flip the impeller, it takes more amperage then whats needed to simply spin the pump. If the supply voltage is low, there may not be enough amperage to overcome the friction of the impeller rotate the motor. With the switch in the drain mode, does the pump hum? Outside of low voltage, it could be a mechanical problem within the pump head or motor. Next, pull the impeller and retest.