It sounds like a connection issue to me too. Making sure the 510 attachment on the mod is not gunked up or pressed down. You could try another tank on it to make sure it's not the mod. If it's not I might try a new coil to make sure it isn't anything to do with your build. You may need an insulator for that mod, or just to unscrew the 510 pin a bit, there is sometimes a mark on the bottom where you can use a screwdriver to unscrew it a few turns to make sure it will attach to the mod.
I have had that problem with one atomizer and it turned out to be an improperly installed coil, though, so you may want to take a closer look at that first (provided you are sure the mod works with other tanks).
Best of luck,
Anna