I encountered that problem in my cannon (but never built a solution) where the magma will occasionally refuse to pump up a shaft, and you need to toggle a door or floodgate somewhere else in the system to prompt it.
Curious. Do you have any idea why this problem occurs, and why that particular solution works, or are we just marking it down as an arbitrary bug?
Rephrasing the problem: if you have a pump above ground that puts magma into an underground hallway network, you can cause magma to erupt onto the surface by opening a hatch on the ceiling of that underground network (search the forums for magma landmines). However, sometimes when you open the hatch nothing happens, even when magma should be erupting. I think this happens because the underground hallways get completely 7/7 full and are marked as "non-flowing" or "static" by the game somehow. Opening a doorway/floodgate in a alcove in the underground hallways causes magma to flow naturally into the space, removing the "static" flag or whatever, allowing the pump to function properly. I guess that the code for the pump checks for the "static" flag, and doesn't bother trying to find a spot to put the magma if the flag normally means that the space downstream of the pump is completely 7/7 full.
Yeah, I used to build massive magma cannons like that. But then I took an arrow to the knee.
MAKE IT BIGGER!
You could build multiple pump stacks in the third cavern to the surface, and multiple pump stacks from the magma sea to the top of the third cavern. Wall off the third cavern, and turn it into a map-sized magma reservoir. Once completely full, bring everything to the surface as fast as possible, covering the map in 3 z-levels thick of lava.