Photo thumbnails not showing in android app

Viewing photos, in folder view using android app displays around 30% of thumbnails. When a folder is clicked on all photos in that folder are displyed correctly and within half a second.
All thumbnails are displayed when viewed using the web app.

For the folders without a thumbnail, if I assugn a poster to the folder then that is displayed as the thumbnail. Pretty sure though I have not done this step to folders that dont have thumbnails displayed.

Using latest pms on rooted shield and android app on shield and mobile phone. Web app runs locally on windows 10 pc.

Fixes tried:
Plex dance on affected folder
Re analyse
Re scan library

Any other ideas or have i missed a bug?

Some more investigation: Editing the folder shows a poster image that is un-selected - see above. Clicking & selecting the image, shows the folder thumbnail in the android app.