Please check the following points.

Check 1
If you scanned using the Brother iPrint&Scan application, the version must be 1.07 or later. Confirm your Brother iPrint&Scan application is the latest version. If it is not the latest version, update the application for the Android™ mobile device and try scanning again.

Check 2
Did you scan the documents using your Brother machine? Brother Image Viewer will only view and edit documents that were scanned by a Brother machine. Also the file formats Brother Image Viewer supports are JPEG and PDF *1.
*1: Secure PDF files, signed PDF files, the PDF/A file format are not supported.

Check 3
The memory where the scanned data is saved might not be recognized.

• If the USB cable is connected to Android™ mobile device, disconnect the USB cable.
• If the SD card is inserted into Android™ mobile device, remove the SD card once and then insert it into the device again.

If the issue is not resolved, turn the power of Android™ mobile device OFF and then turn it ON again.