What is causing my 1/2" plywood underlayment to bubble up and ruin my floor?

In 2002, we added on another bedroom, livingroom, & bathroom to our home near Houston. The house is on blocks above the ground. I installed 2×8 floor joists with support @ 8′ OC, then used chicken wire under the joists to keep the dogs out of the 6" R-22 fiberglass insulation installed between the joists, vapor barrier towards the floor. I then installed 3/4" CDX plywood directly to the floor joists. This was my work surface and we caught some rain during the dry-in period. (about 2 months) Then when we were ready, after texture and painting of the drywall, I installed 1/2" AC plywood over the subfloor to recieve the flooring. We chose an Armstrong peel and stick vinyl tile to lay over this 1/2" plywood. I prepared all the joints and nail holes w/floor stone. The room is cooled by a central A/C heat pump and has been kept around 70 degrees year round. The 1/2" subfloor has bubbled up all over. I have repaired some of the places w/screws. I need some help with this.
Thanks for all the good answers. I did use ring shank nails to secure the 1/2" sub flooring to the floor joists. In the repaired areas, I used drywall screws to repair the bubbles and the plywood still de-laminated. Prior to this installation, my crew laid several thousand sq ft of 1/2" plywood under floor tile and we had no problems at all. There is the insulation vapor barrier that I stapled to the floor joists somewhat tightly. We only dry mop this floor when we clean. I had an architect tell me something about the dew point @ the surface of the floor? I have the same subfloor in my livingroom under a Shaw snap lock type flooring and we have had no problem there. This problem has bugged me for a while.

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