“Update index” command not updating the indexed folder

I have 1 DT3 db where I indexed a folder.
I added 2 folders within this (indexed) folder.
The new subfolder are not visible in DT3.
So I launch a update indexed elements
and the subfolders are not appearting in the dt3 database (under the main indexed folder).
I created a new db DT3 and indexed the same folder and then the new subfolders appear.

It’s inadvisable to index a Finder folder into multiple databases. Also, it’s very likely you have indexed items in the trash of the first database and that trash has nit been emptied.

thanks for your reply. I just created a new dt3 to verify DT could « read » the folder.
Didn’t do any operation like putting in the trash.
The problem I try to understand is why an update of an indexed folder is not updating the indexed folder
And I created a new one to verify (and maybe to replace the old one as it doesn’t update).

Did you add them via the Finder? And where’s the indexed folder located?

Is the indexed folder stored in a folder synced with any online service?

no, the db DT3 is on a SSD drive (directly connected on my MAC, and the folder is on a synology NAS drive

What kind of filesystem does it use?

Is this folder managed with Synology’s Drive app?