Have you checked that the correct coil is installed? (A-1496).

The spring strength and smoothness of the kicker arm throw can have an effect too. If the spring is too firm, there's less 'give' on the arm and the ball can launch up, rather than get a slight 'windup' and launch more forward. PBR has the correct replacement spring.

The kicker should also be a double fork variety for Sing Along (in case it got replaced along the way).

The outhole switch should bend low enough to let the ball rest fully in the kicker arm forks, and not prop or tilt it up.

I'd start with cleaning the kicker arm plates, and have a small dab of SuperLube where they pivot and rub against each other, and then maybe replace the spring (or swap in a less firm one and see how it affects it).

yes, the spring on the kicker arm, not the one on the plunger.

Have you tried the old spring in KoD? (Just curious if the problem follows the spring.)

The spring seems to need to be in the 'Goldilocks zone' to get the best operation. If it's too firm, the fork(s) ride with the initial plunge, rather than the spring absorbing some of that and then flinging the ball as the spring releases the tension.

Would be cool to see it in really slow-motion.