I just recently restored the mechanics on one of these, and I can tell you that the ball lift mechanism has no discretionary way to select whether it is lifting a ball or not. If the machine is:

1) set at the proper slope, and
2) the ball lift shaft is aligned properly with the feeder tray, and
3) the ball lift shaft is screwed tightly at the correct location to the circular disc that cycles the shaft so it can reach the depth it needs to travel to, and
4) it is loaded with enough balls (does not have to be 4 - can be more), then

every time the ball lift mechanism cycles, it will lift a ball to the tee. That is, IF the ball is there to be lifted.

Either there is no ball available for it to lift (because all the balls are in the process of returning, or the slope into the ball lift mechanism isn't right), or the ball lift shaft is slightly off-kilter and needs adjustment. That's my guess.