Why can't I import mp3 files?

When I try to import a group of mp3 files nothing appears to happen. No error messages, but I can't find the files in the RECENTLY ADDED folder.

I have wondered if they don't appear to import because maybe I already imported them years ago and they are lost somewhere amongst the other files (I am searching for them in RECENTLY ADDED).

Or perhaps there is a problem with the files, but why don't I get an error message?

I can't search for the files in the search bar because they are not music files, they all import as UNKNOWN ARTIST and the tracks are called TRACK 1, TRACK 2, etc. I have thousands of tracks just called this in my iTunes.

I've attached a screenshot of the file information just in case someone can identify a problem with the files themselves.

Thanks