This problem usually happens if the phone’s built-in transfer feature (Pixel transfer, Samsung Smart Switch etc.) is used before using SMS Backup & Restore’s transfer feature. Those transfer features tend to create only the text based parts of MMS messages but do not transfer the media properly, leaving a skeleton behind. SMS Backup & Restore’s transfer then detects those existing messages as duplicates and does not restore the remaining parts of those MMS messages.

To fix this problem, you’ll need to delete the existing MMS messages from the phone and then perform another transfer/restore using SMS Backup & Restore.

You can use the “Delete messages or phone calls” feature (Menu > Tools) in SMS Backup & Restore to easily delete all messages.