Incomplete import into MP3TAG

I have about 3700 tracks in my iTunes Media (subfolder Music). MP3TAG reads in only about 3100.
I run MP3TAG on a Mac; version is 2.77.

What could be the reason for incomplete import?

What does the finder say about the number of files in the folders?
Are all the files in the folders you load?
Are all the files there or do you see some orphaned entries in iTunes?
Could the filenames use characters that are not legal for Windows filenames?
Do you have files that do not have the correct extensions for MP3tag?

I had some AIFF files, and switching those to MP3 helped already. Greats, thanks.

With respect to the illegal chars used for Windows. Where can I find a list of those?

See this info from Microsoft: https://support.microsoft.com/en-us/kb/177506