Coil wired incorrectly? broken/frayed coil wires? Plunger binding on coil sleeve?

If the plastic flag that moves out of the opto doesn't fully clear the opto when you push the flipper button it will cause a weak upper flipper. Try lifting the playfield with the prop rod and move the flag all the way out of the opto and see if it's stronger. May just need an bit of adjusting. JEP

Also clean the opto on the flipper board. Another easy test is swapping the left and right flipper boards and see if the issue follows the board.

Had this happen on a Demo Man... I just get a q tip and spray some windex on it and clean the flipper button opto and the dry it with a clean q tip.

Wrong coil size? EOS switch needs adjustment?

Could also be a problem on the Fliptronics 2 board. Check the transistors, diodes, and resistors for that particular flipper. Make sure the transistors are good, measured resistance is in tolerance. If these are all good, you might want to check the LM339's at U4 and U6. The optos on the flipper opto board for the weak flipper may be failing. This can happen whether the optos are clean or not. Swap boards and see if the problem follows the board.

Oh yeah.............check the solder pads for the headers on BOTH flipper opto boards. Make sure none have cracks. If they do, reflow the solder on the header pins.