My friends patio did the same thing. basically he found out that no visqueen was put down under the patio before pouring concrete. The moisture from the ground was coming up through the concrete and getting behind the sealer causing it to become white and flaky according to the builder. There were no fix. He removed all the sealer and just put a new coat of concrete stain on it, which he had to recoat it about every two years to keep it looking good.