Do you live in an area with a strong signal? Try downloading the SpeedTest app and see what kinda speeds you're getting.
For myself, depending on the time of day and using 4g, YouTube videos need time to buffer. However, at night when speeds are quicker because less people are using, buffering is never a problem.
Clearing the cache of the app or phone usually resolves any sort of app issue, but yours seems more tied to a speed/connection issue. Sometimes you may just need to restart the video (I have this trouble at home on my PC even on YouTube once in awhile). Videos will buffer very slowly, but as soon as I hit refresh it won't need to buffer the second time through.