There should be a small amount of space between the flipper and the bushing - about the width of a credit card. Sometimes if the mechanism is tightened too high on the shaft it can cause it to bind.

I actually had the same problem on my ST Prem this week, also with the right lower flipper. I convinced myself that one of the metal parts inside the coil had gotten magnetized which caused it to stick. Replacing the parts fixed it.

I should mention that when I fixed mine I replace the Stern coil stop with a Williams one. The Stern coil stop was loose where the stop attaches to the plate, so this may have been the issue as others have noted.

I only used the Williams one because I didn't have a Stern one. They are slightly different and the actual stop on the Williams is hard rubber instead of metal but other than that the same. After I replaced it the coils did not go to exactly the same height when activated so I replaced the contralateral side with a Williams one also.