Is it binding mechanically at all? When you move it with your fingers, does it give resistance (relative to the other sling)? You mentioned the previous owner replaced the coil. Was it the correct coil?

Normally, you'd start with ruling out mechanical issues...if it doesn't move as freely as the opposite, dig in and find out why. Sleeve, plunger, bracket alignment, bent arm, gummy pivot points.

In your case, if the coil isn't correct and the same as the opposite, that's a really excellent clue.

To test that probability, swap the working coil into the problem bracket.

Also, check voltage to the coil and compare to the working side.

Forgot to tell you to check the linkage. If it is sloppy, especially relative to the working side, you'll need to replace that. Looking at the two linkages on either side, if one looks nice and new(er) and the bad one doesn't, that's another excellent clue that they were both old and sloppy and one broke and got replaced.

What game is this? That will help us all - some games work differently, and then there are different coils as well.

You could probably reuse the old one, but besides being cheap, new diodes are readily available. Go down to Radio Shack and pick up a few 1N4004's. Haven't found a Radio Shack yet that doesn't carry them.

The diode is to protect the MPUs circuitry from the back current induced by the collapse of the magnetic field. Considering they are cheap, best to use a new diode.