Dispensers only fire with non-redstone block update between redstone signals

A dispenser was placed with a redstone on top. When I activated the redstone others shot, but not one. The dispenser only ended up working when given a block update between redstone signals

The pressure plate was supposed to make the dispenser shoot out a block.

It actually didn't shoot until I placed a redstone underneath, but only shot once.

1. Create a Redstone ready superflat with generate structures off and in creative
2. Make a 2 by 1 tunnel above ground. 6 dispensers faced inward to the 2 by 1 tunnel (one facing up, one down, 2 east, 2 west).
3. Put A pressure plate on the dispenser facing upwards.
4. Place a redstone east of the bottom dispenser, another redstone east of that, another east and one block up, the top half of a slab with a redstone west and one block up, place 3 redstone on top of the top 3 dispensers.
5. Place 2 items in each dispenser, step on the pressure plate, you should get 6 items.
6. Step on the pressure plate again, you should only get 5 items.
7. Update the dispenser with the extra item by placing a bock next to it, step on the pressure plate, you should get the last item.