The I/R on the stug works off up left stick so first let me confirm that. Second it works by closing the circuit during gun recoil to fire the I/R. If your not even getting the gun to recoil then your not getting power to the process. I would recheck the solder joints for the wires involved since sometimes they look soldered but it's not done well enough. Then i would check and make sure all the plugs are snugly in the RX18 and facing the correct way. If all of that is fine then it's time to start checking with a volt meter and see where the power is dead.

Oh you can also manualy push the recoil in to close the circuit and test it that way. That will at least tell you if the flash works with power. Tank needs to be on obviously to try it though and the recoil needs to be in it's full resting position.