After building a couple of TNT cannons I noticed that dispensers placed in a certain order (more info below) can get stuck in the "triggered" mode as indicated by the F3's targeted block. This causes them not to fire, and this is quite a hustle for building precise cannons as in cases it may affect different dispensers (as shown in images) making them random. It also may also be related to all dispensers having the same amount of items.

The testing I conducted shows that dispensers get glitched both when shooting out normal items and ejecting primed TNT, not sure for any other actions.

There isn't a problem with the wiring, as far as I know, since it depends on the location, possibly the axis and the way the dispensers are facing/being powered different dispensers may get stuck as shown in the image below. (same contraption facing a different axis and in a different position making a different dispenser get stuck as triggered)

Sadly I don't know exactly what causes this, and have not pin pointed the problem, but I can recreate it with these complex steps:

1. Building the exact same redstone contraption. (TNT Cannon)
2. Placing 1 stack of TNT/Blocks into each of the dispensers.
3. Firing it a couple of times via the stone button on the back.
4. Checking the dispenser's contents to see if they actually fired the TNT/Block.
5. After finding a dispenser that hasn't fired it's contents (has an amount that isn't the same as the other confirmed to be firing dispensers). I can check the block's state with the F3 overlay.
And with that confirm that it is stuck as "triggered".

Since it is quite a complex problem I will provide a downloads for two worlds, "Dispenser Bug 3" having contraptions where ONLY the top dispensers, found next to the shroomlights, get stuck.
And "Dispenser Bug 2" where (ignoring all the other contraptions) you can find the contraption where only the bottom dispensers get stuck it is found under the giant glowing sound making arrow.

This is just a guess but it may also affect droppers.