For the 10K unit it is only driven from the 9 position rollover switch from the adjacent 1000 reel. Inspect that as it is probably out of adjustment or needs the points filed.

As for your left flipper, the first thing I would look at is the coil that is on it. That red wrapper sure does not look like a Gottlieb coil.

To test, if you want to go to a lot of effort, swap the coils side to side and see if the weakness follows.

Otherwise I'd suggest getting the proper Gottlieb coil in there and then it comes down to having the mechanism clean, adjusted properly, and all of the switches in that flipper circuit freshly filed.