Check mechanical issues first. Does it move freely by hand with the power off? Anything binding? Is the coil sleeve good. It should be if it is new, but you never know. Is the flipper attached firmly to the arm that connects to the plunger. There are likely a couple of nuts you tighten with a hex screw. Are they adequately holding the flipper or have they come loose?

I didn't realize there were so many issues with Stern coil stops.

And, now that I see that picture, I am reminded that the flipper is attached with a clamp and a regular nut that tightens it. Sorry, OP, for misleading you about the hex nuts.

I would check to make sure the flipper is firmly clamped in.

Could the plunger be damaged by this? I’m thinking of the fact that the flipper was working for a while and is now not. Wondering if you should get a new plunger as well.

Congrats! A nice story all around, except for the part where they forgot the coil sleeve during assembly.