Things it could be;
Coil could be starting to fail. What are the ohms reading at the coil lugs? After playing for a bit and you notice it getting weaker, lift the playfield and touch the coil. Is it noticeably hotter than others?
Driving Transistor could be failing. Do you know how to test these with a Multimeter?
Plunger, coil stop and coil sleeve could be dirty and/or worn. If it the end of the plunger or coil stop is mushroomed, replace them. Replace the sleeve while you are at it.

I've had the exact same problem on my AFM. In my case the welds of the scoop were all nasty. In fact the whole scoop looked awful. Not sure who welded it but it was a mess.

So I bought a replacement scoop and the problem is gone. All balls now fire on the left flipper, as intended by the designer.