The crossover pieces don’t always work. Occasionally (not that rarely) a ball doesn’t make it quite over the hump and rolls back the other way. I wondered if a slight design change might fix it. I thought of extending the lower ends of the side walls turning them in at the bottom to give the balls a little more horizontal motion, as I tried to show in the image here.
If you haven’t seen it, check out this post:
There is a PDF linked in there with instructions to modify the crossovers. I did it and it improved reliability a lot.