Have you tried adjusting the mod's center pin upward? Sometimes they get stuck. Sounds like it's the mod not the
tank since you've tried several. Has it worked with any others? If you can't get a connection (I believe that tank pins can be unscrewed slightly as well, lengthening them, which may make a connection, you unscrew them with a screwdriver a few turns) but honestly, it sounds like the mod.
Is it a new mod or it just started doing this? If it's new and it's been this way out of the gate and simple solutions don't fix it, I would probably send it in for warranty.
Good luck, maybe others more familiar with this particular mod will be more helpful to you.
Anna