I recently purchased an iphone se (prepaid att). I then received a sim card from att and signed up for a corresponding plan. unfortunately, even though the account showed a positive balance the phone was in 'suspended-mode' (not active). after 2 trips to the att store to troubleshoot this, it was discovered that att needed a receipt/proof of purchase (with the phone's serial number) to release it from the suspended mode.
Not sure if this is typically done at the point of sale, however if not-- to save your customers the hassle--if they should encounter this situation have them either fax their receipt to att prepaid support: 866-246-9528. or email the receipt to ATTPrepaidSupport@att.com
Once I did this my phone was activated fully within 5 minutes.