I have in the past also had LMS library scanning issues – which as it turns out were caused by faulty / corrupted tags. Try rescanning and possibly repairing the tags with Mp3tag (http://www.mp3tag.de/). In my case, some of my files had invalid APE tags.
In Mp3tag, go to Tools – Options – Tags – Mpeg – and under „Read“ select all, under „Write“ select ID3v2 / ID3v2.3 UTF-16 and under „Remove“ select ID3v1 and APE.
Now load all of your music files, select all, and then click the red X („Remove tag“). This will then remove all ID3v1 and APE tags from your files.
Next, select all files and CTRL-S – this will then save and rewrite all tags properly as ID3v2.3 UTF-16.
Finally, in LMS, go to Settings – Rescan Media Library –> Clear library and rescan everything.
No guarantees, but this worked for me. This is particularly helpful if your music files are from various sources with inconsistent and possibly invalid tagging schemes.
PS – further to Heiner’s answer, you can find the explicit LMS media library scan log under Settings – Information (Tab) – scroll to the bottom – Scanner Log File. This will contain any errors the scanner encountered.