Marbles in marble dispenser get stuck trying to fit through hole
It occurred to me that this problem has already been solved...  Gumball machines and/or coin-op toy dispensers at the grocery store are capable of selecting just one of something.  A quick search reveals a solution like this:  



Seem to me you may be able to adapt your design to work something like that.  Even if not, I'd look at gumball machine designs...