There have been a few responses to this in the Subtank threads.
This is most likely a wicking issue. Either too much or too little. It took me quite a bit to figure out how much to put in. If you are using the KGD cotton I use 1/3rd the size of my thumbnail and take off the top and bottom matting before putting it into the coil. It will be tight so twist it as it goes in. Run it back and forth to straighten out the twist and use the "pancake method" to set the cotton. Cut it to where the cotton will just barely touch the deck, then take juice and flatten it both up and down to make a pancake with the center of it being at the coil.
Once I started doing this, I got rid of the dry hit and most of the popping from the coil.