You sound EXACTLY like me months ago. I bottle in EZ Cap swing-top bottles and was having the same problem, except all mine were flat. I tried re-priming, then changing the seals. In the end, it turned out to be the bell (the two wires holding the cap down). It was not holding the tops tight enough. It should be a little work to cap them, and mine had gotten too easy. Here's your fix (probably): take two pairs of pliers and bend each side of the bell a little, just enough to make it harder to close. It totally fixed my carbonation problem. Also, if your seals have been through several batches, change them out.