Coils are not exactly the same length, sometimes the center pin on the mod gets pushed down, and when you change coils the new coil doesn't make good contact. I use a pin or other sharp object to pry it back up. When you do it keep your fingers away from the fire button if using a metal object, or take the battery out if possible. I have to do this sometimes once a week or so. There are some videos on the net explaining how to do it.