There are multiple reasons why all your messages may not be getting backed up:
- If the messages are very long or if they contain multimedia images or photos or if they are Group messages then you need to enable backup of MMS messages. To do this go to the app Menu > Settings > Backup Settings > Turn on MMS.
- If the backups are missing only sent messages then check if you are using Advanced Messaging /Enhanced Messaging for sending messages. You can check this in your messaging app by long holding on a message (that was not backed up) and select Show Details, if it says “Advanced Message” or “Chat” then this is an advanced message (called RCS), it’s not an SMS or MMS message.
At this stage, SMS Backup & Restore can only back up RCS messages sent/received using Google’s Messages app.
If you are using your phone’s default messaging app and you are not using any features offered by Advanced/Enhanced messaging, then you can disable it from your messaging app settings.
Any messages sent after changing the setting will then get backed up by the app.