Or maybe a lot of AP holders did as you and others did, with 4-5 windows open, so the system was swamped. (I'm not trying to criticize you personally, a number of forum members posted about using multiple windows.)
The sign-up for AP previews seems very flawed. Most AP holders do not get the email in time to even know about the preview. People who try to sign up lose an hour or more waiting only to be told the event is full. To me, those are both significant problems.
I think it is high time for WDW to do the AP sign up in a way that is more fair to every AP holder. They should assign each AP holder/family a personal PIN code like they used to do with discount offers.
Otherwise, what? Next time even more people will vie for a preview by having even more windows open, and the problem is just going to blossom into a worse situation.