Viber: message sent but not delivered. Pls help!

Hi my query is that I sent a message to someone while they were offline yesterday. Then an hour later I went back on viber to see if the person was online and if the message was delivered.

The person was back online but the message still said "sent" only, instead of delivered. I can't understand what has happened, why when they came back online the message still was not delivered.

Today, just to see I got my friend to block me on hers and I sent her a message to see what would happen. The message showed up as delivered immediately.

Either way I do not want to re-send another message, I would just like to know if anyone can think of why the message never got delivered even though the person is now on-line. If anyone can figure it out, please let me know as it's driving me crazy not knowing. Thank you.