Another thing to try which is common on Jurassic Park. There a wire loom in the vicinity of the flipper link and it can stop the flipper from responding if it is in the way.

The other easy check is to remove the glass and with the game off, check the resistance of the flipper my moving it up and letting it go. Sometimes the flipper pawl is too close to the bottom of the bushing and causes friction. To fix this, slightly loosen the hex head and tap out the flipper shaft, which will create more distance between the flipper shaft and bushing.

Another possibility is that a previous owner lubricated the flipper shaft / bushing and now it causes friction. To fix this, remove the flipper shaft and clean the shaft and the bushing with a rag covering a thin screwdriver.