Sticking vacuum piston plate

It moved freely with rods installed but before the retaining screw was installed. Gut tells me that the shoulder of the stud is dragging against the guide sleeve in collar on the top of the piston. (The stud that I refer to is in the top-center of my photo). The pin/jet combo were there before I did the rebuild - so they should be the right size. The parts that don’t move freely appear to be shiny clean..but I’m wondering if I need to move a little metal to increase clearance between the two parts that are binding. In the photo... the piston is stuck in the down position. I’m not sure what forces move this thing up and down when things are buttoned up and the car is running - but I’m guessing vacuum from under the piston.