One motor primes pump another motor wont????

I had a motor go out on my pool pump and bought a rebuilt. No matter what I did the pump with the new motor would not prime. I tried everything in the book and nothing until I decided to try the similar motor (1 hp) from my waterfall. Even though they appear the same the older motor primed the pump straight away.

What could the reason for this be? RPM, direction of rotation? When I have the cover off the pump and start the motor it immediatley sucks the water out of the pump so I dont think the rotation is wrong. I'm going to take the new (rebuilt) pump back to the pool store tomorrow but I would be surprised if they can explain it.

TIA, Jeff

That is very odd because priming should not be affected at all by the motor. The two motors should spin at nearly the same speed and in the same direction so that should not matter at all. However, in the process of changing motors, it is possible that you fix and air leak that could have been causing the issue. Also, if there was any debris stuck in the impeller that was cleared out with the second motor change, that could also explain the change.