Interesting project, and welcome to the board!
Not sure what the end project is going to be or why 5 separate slots but I think I've got a rough idea of what you're asking
A couple suggestions. There's obviously a bottle neck at the top where the two marbles bridge across the available opening like this \**/, where you want this \ * / right?. A result of the interface between the horizontal and vertical surface not being wide enough (needs a more gradual taper to the funnel)
Or the 'release' wire shutting the tube too quickly before gravity allows the next one to take it's place. That's a bit more complicated and could require another piece of wire and zip tie, off the solenoid.
If I were doing something like this, a V-shaped trough full of marbles above an auger that spans the five slots, and each 'revolution' on the auger would move a marble horizontally from a 'safe' position (not over a hole), to a released position (over a hole).
Hope this helps! when it's patented for millions don't forget