I've had this problem too; I narrowed it down to the drive shaft nut. It has a "sweet spot" of tightness the user must find to ensure it will allow the propeller to spin correctly and start the pump up, and I'm still not sure what that "sweet spot" is...?
If you tighten the nut too tight, the drive shaft seizes up and the propeller won't turn... If you only loosely attach the drive shaft nut, it tightens up and the drive shift seizes and the propeller won't turn... I took the unit apart and reapplied the drive shaft nut ten times before it finally found its sweet spot and the pump then worked perfectly...
I saw that you, Tim, posted in another thread that the drive shaft nut only needs to be hand tightened, but if I do that it does not work for me. The sweet spot seems to be fairly loose, yet lightly tight, and I am still not sure where that "sweet spot" is...?!
I just keep hoping that everything keeps going well for some time and I don't need disassemble and reassemble the unit any time soon. I don't look forward to finding that "sweet spot" and messing with that nut again...!
In the manual the drive shaft seems to have ridges that fit into the back of the propeller, but my unit does NOT have this--the propeller and the drive shaft just connect flush on the drive shaft screw. I think the ridges look like a good idea, as it would ensure that the propeller is forced to spin...?