I had the same issue but with my entire allocation of adult weekend tickets. Was left at checkout with Green Man Trust donations to pay, but no tickets. Wasn't able to get any more after an hour of refreshing.

Please use a website with a queueing system next time, people can see where they stand relative to others, and have a more assured window in which to purchase. Also means that people don't have to spend an hour frantically refreshing to no avail.