Understanding costs associated with child birth

Had a baby, 2 days in hospital and with epidural. Everything was straightforward. I have a high premium plan ($0 deductible with $1500 copay for inpatient service). I paid my copay to the hospital, and later got a bill from the hospital for addtional charges where the amount billed is greater than what is covered. Why am I being billed the difference, after I already paid my copay?