I have a small water pump (<https://supermagneter.no/vannpumpe-6v-1-2l-min-tilkobling-4-mm-id625>) that I control with a relay that is controlled by the raspberry pi. Usually I can start and stop the pump a few times, but then it suddenly stops moving. I can still here the relay switching but the pump doesn't move. If I switch the polarity over the pump, the pump runs again (in the opposite direction). When I switch back to the original polarity the pump runs again, for a few times before it wont run anymore.

Anyone have any idea what is causing this behavior? I was thinking that maybe there is a position of the rotor for the DC motor where no torque is being generated (or not enough) but that that seems like a very poorly designed motor.

I was planning to use this to water my plants over summer and with this intermittent behavior I am afraid they will die (sob)!