Hi,

First of all, when power is off, does the plunger move smoothly in the sleeve if you handle it manually ? I mean, do you feel a resistance or blocking points ?
Are the sleeve, plunger clean and dry, end stop bracket not worn out ?

Is the behaviour of the lower right flipper 100% ok ?

Have you measured the voltage on the coil lug ? 50vdc is sent to the coil by the flipper board when flipper button is pressed , then 8vdc, for maintaining the bat in up position. Here the 50vdc should make the coil attract the plunger up to the end stop.

If everything goes fine with the lower right flipper, and voltages are good on the upper right one, I guess this is a mechanical problem.

If the voltage is not good, it might come from the flipper board, but lets see ...