One patient receiving SMS hours after it has been sent

Out of all of the SMS messages that we send each day, we have one patient that continually receives it up to 7 hours after the time it has been sent by us. Sometimes it is 1 minute later but normally it is several hours (I have checked on his patient tab under SMS and it shows a history of these message being sent by us at a certain time but in the Status area it shows a different delivery time).
We are not opting for a delayed send so I am not sure why this is occurring with just this one patient. Any advice would be greatly appreciated. Thank you.