Playlists created on devices don't update in itunres

we have multiple devices - iphone/ipods and ipads in our house and we all sync to the same iTunes library. But when we add songs to a playlist and then plug the device into the computer to update and sync with iTunes the playlists don't update to iTunes. They just get reverted back to the previous sync and all the new songs that were added are gone. Is this an issue or just a limitation? thanks!