Could be a couple of things...
Poor connection pops to mind first. Make sure the head is secure in the tank and that the tank is secure on the mod. If those are OK, the 510 on the mod may be dirty or pushed down lower than it should be.
Bad coil, possibly. As juice gunks them up, the resistance can increase.
Depending on the juice, I get about a week out of 30s heads, then they start tasting a little funky, A good cleaning generally brings them back a few times.