drain pump vibrates but does not pump out?

Hotpoint wd64 does not pump out although I can feel the pump vibrating. Removed pump, put in
it on the bench and applied 240v ac (the sticker on the motor says its 240v ac). Looking at the impeller, put a spot of white paint on one blade of the impeller, I can see it rotate approx. a quarter of a turn, then back again, over and over again. So purchased new pump, problem still the same, what is going on ?