All the time, or just when on your data plan/on WiFi?
iOS, Android, OS X, Linux, or Windows, & which version?
Have you tried accessing YouTube in a browser instead of the app?
Does it not even open up to the start page, or it is just buffering when you try to access a particular video?
Have you applied all patches for the app and/or OS?
Have you tried deleting the app from your device & reinstalling it?