If you have a slow Internet connection I suggest you open the page of the video you want to watch, and when it starts to play you click the pause button. Then wait for the grey bar to reach the right end. Then you play the video.
If you experience that the grey bar gets stuck while the video is paused, try playing the video for a few seconds and then pause it again. This will make it start buffering again.
Sometimes it also helps to play and skip forward close to the end of the grey bar. This has the effect of speeding up the buffering sometimes. When the grey bar reaches the end you can skip back to the start and play the video.
This kind of play-and-pause approach is not for watching the video. It's only used to make sure the whole video is buffered.
Also, after watching the full length of the video, don't click to replay it. You just saw it! There's no point in watching it again. It will save you some bandwidth and maybe even Internet cost, and you can go do something else or watch a another video.