iPhone XR files not shown in app

When I initially downloaded the app there were only 986 of the 5817 photos in my camera roll. I could not copy these to the ixpand flash drive go 128gb. Today there are no picture files shown at all. How can I resolve this problem?

I chatted with Sandisk support and received the following info. I hope it helps.
Check if iCloud is enabled as that can stop the transfer from camera roll. If its disabled then try to transfer files to the drive.

Format the drive once on a computer and then try to transfer the files again, In order to format your USB flash drive using your Windows / Mac computer, please follow the steps from the link below:

Formatting a memory card, flash drive or device using a PC (sandisk.com)
Formatting a memory card, flash drive or device using a PC

Note: Formatting will delete all the files in your device. Please back-up all your files from the drive to your computer.