as said, we need more info otherwise weve got around 5000 bits of kit to guess on what the issue is.
kit make and if your not sure post a picture (you may need 2 posts before you an upload a pic)
what coil is in the tank
what wattage are you using
what juice are you using and what is its ratio of vg/pg
have you charged the kit for a good hour or two
As above, also make sure there's no sticker over the contacts for the battery - some new mods are sent out with them and quite a few have been caught out.