it's they way their built, they get a molded peice of foam and stick glass on each side and stick it in a bag and suck the air out until it's cured, then pull it out and sell it. Scince the glass doesen't lap over the sides at all it makes a weak spot along the entire edge. Whats happening is the glass is comming appart at the rail and soaking up water into the foam, which is making it sink, and yea your superglue is probally making it worse by sealing the crack but you getting delam or something and it's just worse by the time it comes appart again.
uhh if you know how to glass it glass around your entire board (on your rails, lapping two inches on each side) with two layers of 6, and hot coat. After you hotcoat and sand, stick tape to your rails about two millimeters above the bottom, the bottom is facing up, so the tape forms a dam around the entire board. Get some more resin that's kicked somewhat hot, and pour it around the entire edges inside the tape, so when it cures theres this sticked up edge that you can sand down and make a hard edge again.
Doesen't make lots of sense, but if can glass yous hould know what I'm talking about, and if you don't, get it professionally reparied.
If they're charging you stupid amounts of money to do it, ($100-200) just buy a new board, it'll look weird and won't last as long as buying a new board.