That was happening to me too. I use the Xyron sticker maker to adhere my image to the bottle cap and then I do a small bead of Mod Podge around the edge to cover the paper edge from the image. I am assuming that the sides of the paper are not sealed and soak up the ET, but I am just guessing with that one.
I just use a small paint brush and dab on a thin coat of the Mod Podge all around the image just to basically seal that edge. It has worked for me and I havent had a bottle cap turn out like that since. HTH!