Add folder to library not showing songs in the folder

I have been adding folders to itunes library but all of a sudden itunes add folder to library function stopped working. I click add folder to library but when I select the folder it says "no items in the folder" as if the folder is blank, no files. However, the folder does have files. At the same time, I open the same folder on my PC using windows explorer and all the files are there.

By using iTunes add folder to library function, I also have looked up the folders I have previously added and they also show empty!! I However, when I click on them in itunes library they play perfectly fine and when I click "Open folder location in windows" it does open the right folder and all the files are there!!!

It's only when I use add folder to library function, the folders are empty. Any idea what's going on and how to fix this? (my folders are on a external drive but like I said itunes had been working fine until now).