Quoted from Daemin:
My pro, fresh out of the box, had a weak lower right flipper. When if it was hit by a ball when held up, it would deflect down and then return to the up position. Most of the time, from a cradle, it wasn't strong enough to get the ball up the center ramp.
Turns out, the little nub of metal on the coil stop had come loose and was floating in the coil sleeve. It didn't stop the flipper from working, but stole a lot of strength from it. Replacing the coil stop with a new one fixed the issue.
So, This is my first post but I want to thank you all for the ideas and answers, and this is EXACTLY what happened to my new fresh out of the box pinball. the left flipper lost strength after something like 30 games. Thanks to the quoted post I found the problem. Now, I got in touch with the vendor but some superglue did the trick in the meantime, at least for the last 5-6 games (5 ball)