"Everything else is working well though the ball usually takes a few tries before it's ejected" I had a super soccer that was like that and it ended up being more mechanical. The bracket that holds the solenoid under the playing field was bent inward from years of use, but when I bent it back out (at a 90 degree to the playing field), it had enough power to eject the ball...just by increasing the travel.

I know...weird solution to a weird problem, but cleaning the contacts didn't fit it, so knew it was mechanical.
Worth a shot!
Joe