by placing 3 dispensers together with blocks on top of the dispensers at least one fails

by placing three dispensers together with a block on top and redstone on the blocks to activate the dispensers, at least 1 dispensers fail. the dispensers contained arrows

What I expected to happen was ...: the 3 dispensers shoot arrows.
What really happened was ...:
one dispensers failed.

Steps to reproduce:
1. put three dispensers together pointing south or north.
2. Place a block on top of the dispensers and place on the redstone blocks.
3. Then walk away making a redstone thread and place a lever.
4. Fill the dispensers with some dispensable items, I use arrows.
5. activate the redstone