The mod itself is shorting out.
(the first place I'd have checked would be the switch, but I think you checked that)
The second place I'd check is the 510 center pin and threading. It's one of the things I hate about the 510 thread system being used all over the !@#$%^& place. The positive center pin is literally only a fraction of a millimeter from the negative 510 threads. It's just the nature of the 510 thread interface which is quite suitable for cartomizer cigalike things but most inappropriate for big battery mods.
(the first place I'd have checked would be the switch, but I think you checked that)
The second place I'd check is the 510 center pin and threading. It's one of the things I hate about the 510 thread system being used all over the !@#$%^& place. The positive center pin is literally only a fraction of a millimeter from the negative 510 threads. It's just the nature of the 510 thread interface which is quite suitable for cartomizer cigalike things but most inappropriate for big battery mods.