We had the same problem; even a minimum-sized exit slot allows movement of the second Frisbee in the stack, which could then tilt into and partially block the exit. In addition, the bottom Frisbee would occasionally prematurely slide out toward the wheel (we have a 90 degree shooter) from vibration or impact. This would allow the second disc to tilt to the back and block the firing mechanism. We solved both problems with a lightly sprung retainer in the middle of the exit slot that swings up and out of the way when pushed by the bottom Frisbee as it exits. The retainer both holds the bottom disc in the hopper and keeps the second disc nested parallel to, and centered on, the bottom one, preventing jams. We have had no jams since installing the retainer. Here's a clip:
http://www.youtube.com/watch?v=2hVpz...e_gdata_player