I have the same problem. Have MB on two PC's, intend to install it on my notebook once I get it working properly on 2nd PC. All are on the same home network. MB installed on the second PC is directed to the library folder on the 1st PC, picked all files up no problem. Also when I add new files to the library folder on 1st PC, MB on 2nd PC picks changes up, no problem. But when I add new files to the playlists on the 1st PC, the 2nd PC doesn't pick up the changes. The .mbp playlist files are stored in Playlist folder in the MB folder. The funny thing is that if I manually add the file to the MB playlist on the 2nd PC, that particular .mbp file on the 1st PC changes the date modified time to the time that I added that file on the 2nd, so MB on the 2nd PC obviously connects to the .mbp file on the 1st PC. Also, when I installed MB on the second PC, it picked up the playlists stored on the 1st PC OK, just won't pick up changes. I think I've tried most options in Library preferences, but if anyone has some ideas please let me know.
Hope that all makes sense!