This is likely because you have iCloud Photos enabled with the Optimize iPhone Storage checked. If this setting is checked, all your photos are stored on iCloud and only low-resolution versions are on your phone. These low-resolution versions cannot be accessed and downloaded so you must change the setting to Download and Keep Originals. After you change this setting, it may take hours or even days for your phone to download all your photos from iCloud depending on how many photos you have. To change your settings, follow the instructions below:
On you iPhone, iPad, or iPod touch:
- Open Settings.
- Tap on Your Name.
- Tap Photos.
- Tap Download and Keep Originals.
Your photos will start downloading to your device but it may take hours or even days to complete depending on your connection and number of photos.