I do know that depending on the bank, the card might show up on Transunion but not Equifax or Experian, or one might show up on Transunion and Experian but not on Equifax. Was this credit report for all three bureaus?
This is uncommon but not impossible for a company that should be reporting to a credit bureau to not. I've had friends lost in the system (one of them had a mortgage lender not report their on-time payments to the bureau until they contacted them). I'd definitely give Citi a call and see what's up.