This was copied from the "Vortech FAQ's" post:
Why is the wet side of my pump spinning?
Sometimes the wet side of the pump will spin slowly in place. This is frequently a sign that the glass needs to be cleaned more effectively on the inside of the aquarium where the pump is mounted. In the event that cleaning the glass does not solve this problem, the problem goes away by itself if left alone for a day or so. In the most extreme cases where this problem does not go away after several days, change the pin spacer setting to one size thicker aquarium. For example, if you have a 3/8†thick aquarium and are experiencing this problem, change the pin spacer setting to that for a 1/2†thick aquarium.
So does this not apply if you have 1/2" glass?...