Study.com

Because the default settings on the website is AUTO-HD, it will change the resolution settings as you go from normal size to full screen, consequently you will lose buffer as the resolution changes. 

You can avoid this by: 

1. Right click (PC) or Double Finger click (MAC) on the video

2. Select either SD ONLY or HD ONLY

3. Play video and Pause. Allow for video to complete pre-load

4. Toggle between the Fullscreen and normal screen to check if Pre-Load buffer is maintained (it should be).

OR: you can change the resolution settings on your browser to avoid this issue. I have safari and multiple programs, so this would be a pain to change the default settings for each program. 

When watching multiple videos, you can have each preload on a separate tab, and do the above steps to each one. This allows all the videos to play as one kind of resolution type, and you will avoid losing your buffer.  This will save you time and frustration if you are in Low-speed internet areas. 

Thank you for providing this work around. I think a lot of our members will find this helpful.