When you lift up on the flipper (90 degrees from the playfield) is there any play or is it tight? There should be a little play and a gap around the thickness of a credit card between the flipper base and bushing . Stern is notorious for installing these with no play and making them too tight. I would also make sure they installed a coil sleeve which they have been known to forget.

I appreciate the offer and glad you were able to find the issue. After being on a few Stern tours nothing shocks me anymore with regards to their quality control.

If you would ever need to add some vertical play to your flipper you just loosen the nut in this picture (courtesy of Wmsfan) and pull up on the flipper. You can see the flipper shaft has plenty of room to move vertically. One of the first things I do when I buy a game is check the vertical play on the flippers to make sure they are not too tight because even though they will still work having them too tight can reduce the strength of the flipper and also how quickly they return to the home position.
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