Solder joints at the coil lugs ( especially the tiny wires off of the coil to the lug ).

Then solder joints at the set of points that fire that coil.

Then check for crappy fuse holder or broken one on flipper board.

LTG : )

I'm going to give you a 'not technical speak' answer, because I'm a newbie too. Both flippers are on the same switch, so make sure something didn't get bent so that contact isn't being made to the outer switch. If you flip up the playfield, you should be able to see the dual flipper thingies. When it is normally working, full pressure will make both go at the same time, while half pressure will just do the bottom flipper. (Handy during gameplay while trying to make the S shot for Chaos.)
I'm looking for a photo to show what I'm talking about and will add it.

Edit: Or listen to LTG. He knows way more than I do.

switch.jpg