Since it works one way but not the other, here's my two cents:
1. Check connections at the switch (for control circuitry): Aside from measuring current/voltage, switch around the connections and see if the issue moves
2. Check connection at the thruster (for control circuitry): same as above, sometime easier
3. Thruster circuitry itself: if you're handy with electronics and understand circuit board, go at it; if not, purchase a replacement circuit board and swap 1 for 1.
Obviously, you got power since one side works. In my opinion, it's not a power issue. It's either a control link broken, circuit board or the electric motor if all fails.
