Did you tap the band clamp as you were tightening it? Does it have a nut on the clamp or a thumb screw to tighten by hand?
Did you tighten it as much as it was prior to disassembly? While my system is from a different manufacturer, I can clearly see how far it was previously tightened by looking at the threaded shaft of the band clamp.
It's probably not tightened enough. With the pump off, try going a quarter turn more. Turn the pump on and see if it stops or at least slows down.