Use an analog multimeter to make sure you are getting the pulse to the coils on the relay as they should be.

The important thing here is figuring out if your relay is failing or is simply not getting correct instruction from the machine to do its task.

Otaku

EDIT: This only applies if it's the two-coil kind. Is it one of those ones with only one coil that locks on via switches? Those are a bit more complicated. The switches on the relay only pertain to locking it on, shutting it off is moreso controlled by the score motor and other switches to disable power to the coil which also disables power to the "loop" circuit that is locking it on.

These are what power and lock on this coil. They are the only things that would cause this problem besides a mechanical, non-electrical problem on the relay itself.

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Lowering the voltage will just do reverse of what you are looking to do, it will give less power to the coil and make it struggle to hold on.

(EDIT: Read this second) You could upgrade the coil to something a little stronger so it really gets those leafs together and in adequate time so it makes contact for longer before it lets go. Remember if you go stronger that these coils that lock on are of a special designation that does burn up when locked on.

Also, have you cleaned the playfield switch and adjusted it? That's probably it! Remember, it's not only a switch but is the thing feeding the initial power to the coil. Dirt will limit the power, and the adjustment will affect how long it sends voltage to the coil. If it is just tapping it at far distance, it's probably only making contact at the exact apex of the ball rolling over that switch piece, which would explain a lot for your situation here. If you adjust it closer, it will make contact as soon as the ball touches it, and will not let off until the ball has not made contact with it.