Every hotel within the Hilton Brands (Hilton, DT,ES,Hampton etc) have a revenue management system that looks at trends, prior year info etc. and makes recommendations on how many rooms a hotel should overbook, what rate levels should be charged, what minimum night stay requirement to put in place etc. - much like what the airlines use. If a hotel is saying it is full - chances are it is and highly likely overbooked. The other possibility is there is a minimum night stay requirement in place and your reservation does not fulfill the requirement - you want a one night stay and the hotel is only accepting reservations for three night stays for example.