Force generating thumbnails on DiskStation DSM

Question

I would like my Synology DiskStation to generate thumbnails of pictures present on specific folders.

DSM version: DSM 6.0.2-8451 Update 9

I set up Media Indexing that way:

Control Panel > Indexing service > Media Indexing, then click on Indexed Folder, and add my folders with Photo checkbox ticked. My 'thumbnail quality' is set on Normal Quality.

I am adding pictures to the server via SFTP or such non-software related medium, and no thumbnails appear to be generated (only one image has been added, and no thumb after 24h). The only way it generates the thumbnails is when I manually launch Photo Station and browse to those folders which is NOT the way I want (plus, that way seem to have the following flaws: it doesn't manage all the jpgs I have, even in the same folder (why??) + it uses a compression algorithm which doesn't compress much (a 1:4 thumbs have half the size of my original (thumbmail quality setting: Normal Quality)).

I read everywhere people wanting their Diskstation NOT to auto-generate thumbnails, but I want it TO. Apparently, CPU-use is the main reason for people to get rid of it, so if there's a light way to do it, it'll come first.