Not doing anything wrong - you are just competing against hundreds of others.
I've never failed in the past...I make sure I am on my fastest connection, be ready five minutes before store opens and refresh repeatedly when it is due to open. I make sure autofill is going to help on the name/address/credit card details.
I don't think there is any secret...I think you've just got to work faster.
My last purchase was probably a year or so ago, so if it has become even more popular, like concert tickets, with so many people there might be a luck element too.