So it seems that when an Atty was pushing down on the 510 pin on the Mod, it was pushing down on the silicon tube that connects the Squonk tube to the 510. This wouldn't really be a problem but in my Mod there is some glue (possibly accidentally) partially sticking that tube to the back of the button, hence as the atty gets screwed on, the button gets pushed down, jamming in its hole. I got rid of that glue and it all seems fine now.
Just in case anybody was curious, you need a T6 Torx bit to remove the 2 screws on the top of the Mod.