Missing or Incomplete Backup
They have not been found on your Cloud storage. This means they might have been deleted, or their upload was interrupted. If you have your photo backup set, please make sure you are connected to the internet and your photos are all uploaded before you close the app.
Your Phone Storage is Full
Another possibility is that the photo is not correctly imported to the app. This might be the case when you hit your phone storage maximum. Backups generally small files, but your phone might be already full. If you have not enough space on the phone, try rebooting the phone or clearing the cache (temporary files) from apps that store most content. Android and iOS have both tools to effectively and selectively clear important storage space.