i've had this happen with a couple of mods, and found that you can sometimes fix it by
a: cleaning out the 510 connection with a cotton bud and some iso alcohol or something,
b: gently pushing the pin in the mod a few times, up and down.
the spring loaded pins can sometimes get stuck. also, try screwing your tank on really slowly and watch the resistance reading on the mod, to see if it's getting a good connection on the way in, but shorting when it's fully tightened. i've had this happen as well.