Here's where my problems started, when I had moved music from one computer to another:
I exported my playlists on the old computer, then moved the playlists and my files to a new computer. Once MusicBee was installed I directed it to my music folder, then imported the playlists. This is what was confusing: my main music folder would play all of my music with no problem, but the imported playlists said that the file locations were missing. But, I noticed in the missing location pop-up notice that the directory was different. This is because the new computer had a different user name in the C:drive. This is why the main library would play music, but the playlists directory was not finding the files. The playlists was still looking for the old computer's directory.
Here was the fix:
I went to the MusicBee playlist folder and opened the playlists in Notepad++. Use the Search>Find... command, click the Replace tab. Put the old directory location in the "find what" window and the new directory in the "replace with" window. Click "replace all". Save. If MusicBee is open, close it and open again. Give it a moment to read the new playlists directory.
Hope this helps to solve your playlist problems too.