In some fo our cruises the balconies have sold out months before the sailing date.
We did get a an up-grade once from an ocean=view to a balcony.
Since then we allways book one to make sure we get it.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Independence of the Seas 12/29/2013
Booked Allure of the Seas....12/27/2015