SUMMARY
After adding 70.000+ images stored on a NAS in several folders representing the day of the shoot the thumbnails wasn't generated before I entered each folder in the album in the thumbnail view. A very slow proces. Then I tried to force the generation of the thumbnails using the Tools > Maintenance functionality, selecting a parent folder result in no thumbnails generated. Unfolding the parent folder and selecting each single sub-folder resulted in generation of the thumbnails and fast thumbnail showing when browsing the album.

The fix is to ensure all subfolders a selected, when selecting a parent-folder (which is how it should work from the beginning and what makes logic sense). 


STEPS TO REPRODUCE
1. Add new album with several folders all containing images: 
New album
 > Folder1 
   > Sub-folder1.1
   > Sub-folder1.2
   > Sub-folder1.3
 > Folder2 
   > Sub-folder2.1 
   > Sub-folder2.2

2. Go to thumbnail generating within Tools > Maintenance.
3. First select the new album and choose to generate thumbnail.
   Result: only thumbnails for that specific folder is generated
4. Secondly unfold all folders in new album and select each of them followed by generating thumbnails.
   Result: Now all thumbnails is generated.

OBSERVED RESULT
Step 1-3: No thumbnails is generated. Going into on of the folders in the album in thumbnail view result in slow appearance of the thumbnails (they are generated on the fly).

EXPECTED RESULT
I expected the thumbnails to be generated and readily accessible in the thumbnail database and therefore showing fast in thumbnail view. That is the case step 4.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 11.2.2
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
It is DigiKam 7.2.0-rc.