The players serve the quality rendition of the videos based on the device type and window resolution (phone, tablet, computer, tv) and network bandwidth/conditions. The player tries to give the best experience (both in video quality and fast video loading time) this way. They detect dynamically the bandwidth and the window size of the player and they serve the most suitable rendition.
Usually when this kind of thing happens it is some kind of a compatibility problem with the device or browser, or there is something wrong with the internet connection.
You could try one of the following solutions that have fixed these kinds of issues in the past:
change the quality of the video to a lower setting using the small quality selector cog in the bottom, right hand side of your screen. Choosing a lower setting usually fixes any loading or freezing problems.
Try a different device or browser.
Reboot your device.