iTunes for Windows wont always import complete folders

Hi

Running iTunes 12.13.4.4 on Windows 11

My media files are held on a NAS and I copied the whole library to my laptop (I know I could point to them in iTunes directly but doesnt help me when I am travelling plus the big issue for me is that I DO NOT want itunes to manage folders and files on the NAS as it changes stuff about which then wrecks the folder structure for my wife who uses a different media player to access the same NAS)

The folders and files are a mixture of all .mp3 files, all .m4a files and some folders are a mixture of both .mp3 and .m4a

The problem .....

In iTunes I select add folder to library to import - in this case the whole library - and it starts to import lots of folders. BUT some folders are imported fully (i.e. all songs irrespective of file format), some folders only import the .mp3s and miss out importing the .m4a and vice versa - some folders have imported the .m4a and missed the .mp3

I then tried narrowing down the import - so just importing albums from one artists - same thing happens

Then tried importing just one album - again, the same thing.

iTunes always seems to be consistent with which tracks from each album it imports. So its NOT like a random thing where it skips different tracks from different albums each time.

At the windows level I have checked that files in the folders exist (they do), are not hidden (they are not), dont have any strange security settings (they all seem to be the same as each other)

What I CAN do however is to use the file add file function where I can browse to an already partially imported album, select the missing files and import them with no problem. Problem is I have some 17000 tracks and it'll take a long time I think to go through every single album imported, cross reference the windows folder to see which tracks (if any) are missing

I have tried dragging and dropping missing tracks/albums directly into iTunes screen but it seems to have made its mind up that some tracks it just wont import (unless I use the file import

Have scouted the Internet and have tried lots of suggestions but no luck.

By the way I want to continue using iTunes - my iCloud drive is already full and to load all the media will be expensive in disk storage plus I use iTunes to take regular backups and images of my various Apple devices and store them to local hard disk/external hard disk and NAS

Any help or suggestions would be most gratefully received

Thanks and regards