minecraft-java-edition – How to Safely Remove TNT in Minecraft

Redstone wire is a little tricky with how it can and can't turn. Replacing the sandstone under your TNT with more TNT and running the wire straight up to that should detonate it.

Alternately, elevate your wire to the level of your TNT before it reaches it, and have the wire run straight into your lowest TNT block.

I just re-watched the video, and compared it to what you are showing us here. I think I may have figured it out.

The video instructs you to place the final 3 bits of redstone on the second set of dispensers while the clock is running. This creates a delay between the explosion of the projectile TNT, and the propellant TNT. If you already have redstone on top of the second set of dispensers when the clock starts, the projectile TNT will be dispensed at the same time as the propellant TNT, and go off at the same time as your propellant. That little delay is critical to ensure that your cannon doesn't blow up in place, and that your projectile will actually move forward.