With the glass off, flip the left flipper. When it’s stuck in the up position, lightly push it down, or, as an alternate, turn off the machine when it’s stuck. If the flipper drops down when powers off, the problem is electrical. If it stays up with power off, it’s a mechanical problem.

As far as weak flipper goes - clean the switches that the flipper pawl hits when it’s energized. Do this by pulling a business card thru them while holding the switch closed. Do this with power OFF.

Report back.

It's mostly likey getting mechanically bound on the coil stop (L-bracket at the end of the assembly) and the vibration from flipping the other flipper moves it enough to unstick it. Look into new coil stops, a second spring, or better alignment of the parts if its mechanical as suspected.

It sounds like you could use flipper rebuilds all the way round.

Could be a megnetized coil stop.

The tell is that activating the other flipper makes it drop. In my experience anyway.

Regardless as others have said you probably need to rebuild your flippers anyway.

Make sure nothing metal, including the apron is touching any part of of the flipper switch or a flipper will stay energized. It is often overlooked.

It’s definitely the coil stop. Replace it and you’re good.