Each time the sending job tries to send a message and the carrier rejects it, the Error Count increases and the error text is stored (open the message to see "Error: ..."). Sprwt retries on later runs but gives up after 3 failed attempts — the message then stays in the Sending tab and is no longer picked up. Typical causes are an invalid or unroutable phone number, a number not attached to the messaging service, or a suspended SMS account. Fix the cause, then re-send the message (for a single customer, from Chat); stuck rows can be deleted from the Sending tab.