Arrows fired from dispensers don't hit entities directly in front of the dispenser

Edit by Zeb: The bug seems to be either that the arrows are spawning ahead of the entity, such that the arrows never actually hit it... OR that the arrow never actually penetrates the entity until after it goes past the point where it collides with the block behind the entity... it seems like removing the block that the arrows get stuck on causes the entity to get hit, at least in my testing.

Original Description:
I tried to build a witch farm with a tripwire hook at the bottom and a two-tick pulse generator hooked up to some arrow filled dispensers. The witch triggered the dispensers however did not take any damage from the arrows!? I am very confused. I will test this with other entities. This could be the witch drinking an overpowered potion or a problem with the arrows registering under a pulse. Please help me as this farm is really necessary!!!