Sounds like a credentialing issue, or wrong patient status
Exactly, if they were inpatients, it sounds like you should be able to bill for subsequent hospital care codes 99231-99233 unless another doctor of the same specialty/group sees the patient on the same day and also bills for it.
Is this happening for all providers and insurances, or is the problem isolated to one physician or one insurance? I wonder if it's a credentialing issue, or if your contract with that payer needs to be updated to include the subseuqent hospital care codes on the allowed schedule?
Of course, as Louise said, if the insurance has no record of the patient being admitted as an inpatient (say for example if they are under observation) and you are billing inpatient codes, they will of course deny those claims. Make sure you have the patient's status correct.