Unseen folder in HDD mounted on router

Volumio Information

Volumio Version: 2.915
Hardware: Raspberry p3 B+
DAC: Ibasso dc3

Hello everybody,
this is my first post, thank you in advance.
I have installed volume on an rpi 3 that reads an hdd mounted on the router.
The HDD has many files (about 33000), some folders copied with samba protocol from windows or android are not seen in the file scanning process.
I tried to copy a folder from both windows 10 and android but both are not seen. Both contain flacs to which the right tags have been put with Mp3Tag from windows and with the Autotagger app from Android.
I have tried the scan several times with no success.
Does anyone have an idea why?