Here's the problem:
You have several different playlists.
You open up one. Let's call it Jack.
You want to add a new song or two to Jack. You do so.
You save the "new" playlist as Jack again.
You go to work.
When you come home and open up Jack, he is EMPTY!!!

I confirmed this problem with one of your monitors. What you actually need to do is: every time you add new songs to an existing playlist, you must save it under a new name. The original playlist is wiped clean! Then you have to go into your program files and delete the old playlist before you can use that name again. Very annoying and I can't figure out why more people haven't complained about this.