You don't say what kind of tank and mod you have. The most common solution to your problem is probably the center pin on the mod isn't high enough, if you use a pin or toothpick to pry it up a little bit it will probably help. If you use a metal pin or needle to pry it up keep your fingers away from the fire button, or remove the batteries if possible.
I guess you did say what kind of mod you have in the title.