Similar issues to some others on here. Managed eventually to get all required tickets EXCEPT the all important (for us) Tribune 18. Availability was showing for the tribune tickets but every time we tried to select our preferences the site responded 'your selection exceeds the current availability' which was odd as the availability was showing in the high hundreds and I was only selecting one! After numerous attempts I gave up and checked out with what we did have.
I then emailed the ticket office stating the problems we were having. They, to be fair, responded quite quickly with an old style order form showing our already ordered items, along with the 2 required tribune tickets. We have therefore secured what we wanted, but not by straightforward means.
Friends of ours had no problem at all and we were applying at much the same time!
Go figure......