Ok, there is a fairly long path for the flipper circuit so you need to break it down.
Check with a DMM for flipper power at the flipper switch contacts. Do you find it at one side of the flipper switch? If yes, then ground it with a wire - it should fire. If it does, your path to ground needs further investigation.

If you have no power at the flipper switch then you need to find out why that is not getting there. basically, you need to see power at one side of the flipper switch and the switch completes the circuit. So do the check I suggest and then we can see which "half" of the circuit needs further troubleshooting.

Meter needs to be on DC volts, if it is not on auto ranging set it to any setting 75VDC or above. You will be looking for 50-75VDC on your reading. Attach the negative probe to ground (usually the ground braid, unless it is broken somewhere) or a backbox screw. It helps if your ground (black) lead on the meter is an alligator clip instead of a probe, because clipping it on frees up your other hand. Then touch the positive probe (red) to either wire lead going to the cabinet switch contacts. One of them should have around 50 to 75 volts DC on it, according to the schematic diagram for Whirlwind it would be a blue wire with a grey stripe (BLU-GRY)

My LAH had a dead left flipper and everything checked out fine. Turned out that the metal contactors that touch when the flipper buttons are pressed needed a light sanding.