If you enabled iCloud Photos and selected Optimize iPhone Storage, it may result in only thumbnails stored locally on your iPhone.
If you want to export the original photos from your phone to your computer, you need to make sure that you already have your original photos stored on your local device first. Please follow the steps below to operate:
- Go to Settings > [your name].
- Tap iCloud, then tap Photos.
- Select Download and Keep Originals.
After the photos have been fully downloaded to your device, you can try Mobiltrans to export photos to your computer again.