Cannot add piston head to piston

I created a 30x20x1 trench in the voxel ground. I had used 4 groups of 5 stacked pistons to create a working pocket door with blast door blocks for a hangar on the planet. There was a lot of strange tension in the blast door blocks, and I know some voxel earth was just bleeding into the vertical space of the trench, so I removed a blast door block to remove the dirt obstructing smooth movement and the blast door and pistons caved into the trench. Before I did this the stacked 5-high pistons expanded and contracted fine, there was just a strange tension in the door blocks. (they vibrated a little, and some of the edges at piston boundaries weren't perfectly connected. I cleaned up the mess, removed *everything* from the trench, and outlined the 30x20x1 trench in light armor blocks, so I could visibly see any voxel encroachment. There was a little, but there is none at all now. I am now trying to build new pistons in the now completely clean trench and I cannot put a head on a piston on top of another piston anymore. Clicking 'Add Piston Head' on it does nothing. In none of the 30 blocks along the trench can I now add a second piston with a head on top of the bottom most piston. This previously worked, and I do not know why it does not now. I've gone along the walls to make sure there are no 'bits' of voxel, and no bits of broken steel in the adjacent blocks. I don't know how to illustrate this visual to attach an example, so I hope the words covered it. Let me know if I can do anything to help further troubleshoot this please. It's absolutely destroying my motivation to continue working on the world I've put a lot of time into. This is a MP SE server I run and play with one other player.