Dispenser to hopper function

I have a farm setup using a redstone mechanism that sends an item from a dispenser to a hopper which feeds back to the dispenser.

 

On occasion when the dispenser shoots an item the hopper does not get it.  Which in turn breaks the farm until manually restarted.

 

I at first thought it was the glass blocking the item on occasion, but it still occurred when the glass was moved.

 

attached should be an image