For the experts:
I once had the same experience on united.com. (2011) What they tried to claim was that a booking and payment did not actually complete the transaction, the ticket had to electronically issued. However they could unilaterally choke the system to delay the the issuance of tickets to give them this excuse. I got web support to go back and issue the ticket as of when I paid for it.
Does the OP have a ticket number ?
NB there is mandatory compensation for "overbooking"
http://airconsumer.dot.gov/publicati...tm#overbooking