Check here for service status of the Yousician apps. We keep this article updated if there are server issues or major bugs affecting the apps.
Video out of sync or stutter on PC/Mac
If you see the background stutter when playing a song, go to the Game tab and toggle on the Fullscreen option. Change the Display resolution to a lower setting to help reduce stutter. Once done, you can toggle off the Fullscreen option and play in windowed mode.