EDIT: Oops, I assumed you were running MC23.0.48 because I just read a thread where someone confirmed they were. Doh!
As you are posting in the MC23 forum, I will now assume you are using MC23. What specific version? Update to MC23.0.48 anyway. There was a problem fixed with Auto Import in MC23.0.47, although that stopped all importing. This could be related. See
https://wiki.jriver.com/index.php/Release_Notes_MC23Otherwise...
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My sequence of actions if this happens is;
Always check the "Detail" at the end of a manual Import.
Check the "Recently Imported" Smartlist and if there, add the [Media Type] and [Media Sub Type] columns and check they are correct. The "Recently Imported" Smartlist is searchable like any view, so search for "Montrose" in there. i.e. TV Shows sometimes import as Movies, and so don't show up in my Shows View. With Audio that is less likely to be the problem though, unless you have custom Views that exclude some files. Unfortunately if this isn't the first attempt, the data may have "fallen off" the Smartlist by now. It doesn't keep them forever.
Check the "Removed Database" and if I find them in there, delete them from that list. Rerun Auto Import.
Check tags in the original files with Windows Explorer or some other tool. Check the spelling of important tags when doing this. i.e. "Montrose"
Turn on MC logging and run an Auto Import, and try to work out what happened. I find
Notepad++ is an excellent editor to view logs, search for content and so on. Search for part of the file name to find what is going on.
If MC plays the files, they are okay. If it imports them when you play them, then it can import them. It is most likely a View that is hiding them, or they have been previously removed, so they get removed again. I wouldn't be surprised if the "Import on Play" function ignores the "Removed Database", but I am a bit surprised that the files get removed from the Library afterwards.