Simply put, either geometry or tension (provided the belt is good).

Either the tension on the belt is incorrect (or slipping) or one of the components crooked (pulleys crooked or bent or the whole power steering pump crooked).

The tension is kind of self explanatory. I check my geometry with a T straight edge that I make. Basically I put a piece (usually wood) on the end of a straight edge at 90* and then lay the straight edge against the pulleys to eyeball for gaps. I guess that's not the greatest explanation but the drive is to check and see that the pulleys are in line so the belt does not ride off.

MB