This can be due to various causes.
The image passes through three different filters in the image extractor, which are decisive for whether image content can be detected.
1. When recognising image files, the image extractor may recognise several contents within the image and the image is segmented into different zones, so-called slices. These slices are then filtered out.
2. The width and height of an image or other graphic elements must have a minimum size in order to be recognised by the image extractor.
3. The ratio of image size to file size must meet certain minimum requirements - otherwise it will be recognised as insufficient and filtered out.
(This filter is currently deactivated - as of April 2020)