I tried calling earlier this evening. The castmember w/ whom I spoke was just as confused as we are... she couldn't figure out why there aren't any earlier Garden Grill times.
I'm looking for something earlier on 9/12, and, at first, she speculated that maybe the restaurant is booked earlier for some sort of special event on 9/12. I told her I was a member of a Disney messageboard & that people were reporting that they couldn't get ADRs earlier than 6:30 for the entire week. So she looked & agreed - there's nothing available at Garden Grill earlier than 6:30 pm & she had no idea why. She actually used the words that the "Garden Grill is blocked out until 6:30 pm."
She told me to keep trying & that the earlier times might open up.
Maybe someone else will get a different response or maybe talk to a castmember that has some inside knowledge!
It really makes me wonder what's going on w/ Garden Grill for that time period...