Also check the switch wireform, to make sure it's dropping low enough to allow the ball to rest flatly against the kicker arm's face.
If it is, and it's still missing a lot of the time, then replace the kicker arm and pivot bracket, there's enough play in there to cause the arm to move.
Once it's all tight, the rocket kicker should hit the hitchiker 100% of the time.

Edit to add: If you cannot find a proper kicker arm (A-16714, 'V-Pad Crank Assembly') and instead use a slingshot arm (A-12664, 'Kicker Crank Assembly'), be sure to dremel off the top lip of the arm.

Rocket kicker? Into the hitch hiker?

The two screws on top of the playfield holding the plastic piece down right at the rocket launcher also give you a LARGE amount of adjustment for the metal plate at the rocket launcher.

Two times I've enlarged the slot in the metal plate with a needle file to get just a bit more adjustment, but alignment with the hitch hiker shot is mostly the metal plate.

Level first. This is one of those adjustments that can 'dial in' a machine and make for an extraordinary play... or an average playing game. So level the machine properly, then adjust the plate.

If your machine is quite a bit out of level, and the plate at the rocket launcher is quite a bit out of alignment I would expect a lot of 'bounce outs' and missed hitch hiker shots.