To ensure your upload is of the highest quality, make sure in the settings of Facebook, under Video Settings, "Upload HD" is selected.
Settings: Navigate to your Facebook settings and privacy, then to settings.
Media: Select the "Media" option under the preference section, then go to "Videos and Photos".
Video Quality: Make sure "Video Quality" is set to "Optimized".
When you initially upload a video, it takes time for FB to post the high-res version of your video (especially if it is larger in size)- it’s basically processing in the background. Facebook will give you a lower resolution version to view in the meantime, making it seem like your video has not uploaded properly
Sometimes within FB, with a larger video, you need to press the tiny "HD" watermark on the video to actually see it in HD.