mp3 music playlist not syncing with apple watch

Both my apple watch and my iphone are updated to the latest OS. I am trying to sync my mp3 music playlists to my apple watch via watch app. But whenever I select the playlists I want to sync and press + button on the top right corner, it looks like it's syncing, but after a few seconds, it disappears and nothing happens. I don't know if it's my watch problem or the watch os bug.