The 510 connections on either mods and atties are not standardized.
Some 510s on mods are deeper than others and the positive connection pin might have more or less travel.
Same goes for atties. Some of them have a long protruding centerpin, others might even have a recessed one, and even the negative threads might be longer or shorter.
So couple a mod with a deep 510connector and not much travel with a short 510 on the atty, and it just won't make a connection.