Since tapping the pump seems to get it started for those of us with this problem, it sounds like the impeller is getting jammed somehow. I can only think of the following things causing this:

Screwing the screws down too much?
This is the easiest to test, just adjust the screw tightness and see if it changes the chance of starting propelry.

Maybe rotating the top is causing the issue, is the center of the impeller exactly centered with reference to the housing?
To test this, we need to rotate the housing an dsee if this fixes or causes problems. Does anyone have a picture of the stock orientation?

Perhaps it's caused by the plastics sticking together lightly.
To verify this, we need to know if increased temearatures increase the chance of starting properly.

Can someone test these? my pump is pretty integral to my system's operation and i can't really test it myself easily.