A while ago I had the problem of having tracks missing in playlists (track count wasn't the same as exact playlists on the computer) even though I copied the entire music folder to the phone maintaining the folder structure and editing the playlists in Notepad++ to change the paths accordingly. So I tried a bunch of different ways and here's what worked for me.
Have your music organized, you'll have less headaches.
1. Make sure your music is copied on the Phone/Tablet first. Adding new files? Be sure to move them to the device before the updated playlist.
I copied my files with Windows Explorer since some tracks weren't being copied with Winamp.
2.Save playlists as *.m3u8 files (I used Winamp for this) and rename them as *.m3u so Android can read them.
3. Edit the playlist with Notepad++ to batch replace paths eg."\" with "/" & "/Users/*USERNAME*/Music/iTunes/iTunes Media/" with "/storage/external_SD/"
I also like to get rid of the lines starting with #EXTINF, but that might not be needed.
4. This Might not seem very important but I put playlists I'm copying in an uncompressed .zip file since for some reason my computer copies them faster than .m3u files. This can also be helpful because playlist issues can be hit or miss. If the track count is wrong just delete the file and unzip a new one (worked after a few tries).
There's no guarantee that this will work 100% of the time but it's been working for me up until now. I'm posting this because I've looked around & didn't find much help.
Tested with GoneMAD Music Player & Default LG Music App (on Lollipop)