Sticking flippers are often caused by lack of “end shake”.
Too much or too little and the flipper can stick in the upright position. It can also be commonly caused by the Stern pinball end stop 515-6308-01 failing and it should be replaced. Look for “gold dust” in the cashbox, thats part of the brass on the end stop coming apart.
To check for end shake on your pinball machine, take the glass off and get hold of the flipper cap, see how much it moves up and down (not the way it hits the ball), it should be around 0.5mm – 1mm. You can adjust this by losening the crank using a 5/32″ Allen key below.