Concrete board is a great underlayment for the tile walls - not for the floor. I have a flat roof over my carport which is also a sun deck. I used a product called Deckcoat over CDX ply. I have explored the option of using Gecoflex, but it is very expensive. David A Waterproof decking systems require top grade material and workmanship.
Most decking products specify a high grade plywood as an underlayment.
CDX plywood was most likely the wrong material with the last deck. Make sure that you use the exact grade of underlayment specified with any new deck coating. In addition, follow all manufacturers guidelines about nailing, flashing, fill material - the devil is in the details. I have used Spandex and Gecoflex, both are good products. For more information, see Decks .