The switch for the rollover button may be too stiff, a ball shouldn't be able to lodge on top of it. You have a soft touch nudging, though.
I've not seen this with a button, but both the Stern 'Ali' machines that I owned had a rollover wire a bit too tight. At the same location, too, maybe someone on the line was having a bad day (they weren't far apart in serial numbers). The ball sometimes lodged on this rollover, and gentle nudging could repeat the scoring awarded from it, which could be a tidy amount (1,000 plus 1,000 for each letter lit of a spell-out feature).
After making sure that the switch is firmly attached, I'd reduce the tension in the longer of the two switch blades (the one making contact with the button), so that the ball can't catch on it. With the power off, of course. It still needs to be tight enough to hold up the button insert, and not move under vibration, but not so tight that it can hold the ball on the button. If the switch is then too closely gapped, adjust the shorter blade to suit.
To check on adjustment in the switch test rather than play, you'll need to have the switch matrix clear, i.e. the ball in hand and all drop targets raised. The number assigned to this rollover button is '34', there are two more switches with the same number/wire colours; the r/o button for exiting the Waterfall upwards, and a rebound behind the Waterfall drop targets.
