In some cases, SMS carriers may simply report "Prerouting Complete" for some time after sending a text, indicating the message has been received by the carrier routing network, but no further status is available. Such texts are likely delivered without a return receipt or the return receipts are still in processing by the carrier network or gateway.
SMS receipts are not guaranteed to be returned by mobile carriers in "real-time". Some carriers send them right away, but many send receipts in batches over time. Carriers prioritize delivery over status reporting, so return receipts can lag significantly behind the actual delivery time for a text.
If/when the carrier network sends a receipt, Omnilert will update the status for that message accordingly.