I agree with Rick that the persistent bubble problem may well be caused by a leak at the end cap/ gutter junction. If it were me, I would re-seal that whole area. I have removed windows that I thought were the problem just to discover that the source was really the joints higher up. The intruding water will find a path down as far as gravity will allow it and either escape or be soaked into the wall if it can't drain out.
__________________
|