There are two reasons why your photos may have not synced from FTB to your online tree.
1) Either your settings were set to sync only specific photos:
To check if this is the case, open the project in Family Tree Builder. Go to ‘Sync’ > ‘Sync settings’:
Choose ‘Advanced Settings’:
Choose to sync all photos:
Click OK, and sync your project again.
2) The other reason can be that that specific photo has been deleted from the family tree online:
If you deleted a photo from your family site, then in the next sync the Family Tree Builder will not sync the photo again.
In order to re-sync this photo you will need to open the project in the Family Tree Builder. Go to the Photos section of your family tree
Search for the specific photo you would like to re-sync and double-click on it:
Un-check the option ‘This photo has been deleted from the family site’:
Sync the project.