YouTube videos may be only available in certain countries, or you’ve exceeded your viewing limit. You can try using VPN app for loading your video from a differen country.
Consider using a different browser. One option is to use Google Chrome which does not have this problem, or another popular alternative.
First, try to lower the resolution of your YouTube video below 720p.
Second, Google Chrome will likely block autoplay on a lot of sites that have user account login or auth. Viewing videos in this case requires manually clicking play and not refreshing the page while viewing.
Thirdly, make sure you have an up-to-date browser.
There are a number of possible reasons for this. It could be your internet is lagging, or the video you’re trying to is not loading because it’s been deleted by the owner.
YouTube has also slowed down their videos due to copyright restrictions, so it might just be that these types of videos are not being shown on your web service.