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.