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The issue relates to the discovery and indexing process failing to find the images on the phone.
A restart\reboot of the phone can sometimes resolve the issue, however if after 5 minutes the images are still not showing you can find them manually by going to the 'file browser' and finding them in the appropriate folder within the phones storage.
To resolve the issue, a factory reset would resolve the problem, but then you have to load all your images back on, another solution would be to download an app which helps discover files stored on the phone.
Scan Media - Android Apps on Google Play