The deck beneath the stain was not properly cleaned. There is old sealer or oil of some kind that is preventing the stain from penetrating the deck. I wouldn't really worry about it this year, but plan on restaining the decking next year. Find a Sherwin-Williams dealer near you, and get some Stain and Sealer Remover, as well as some Revive. It's the best product on the market for stripping and cleaning a deck. Let it dry and restain, and it will look great and hold up. Hope this helps.
