It's the hashrate which is an encoding thing. I noticed when I uploaded previously all hashrates on 720p or higher from youtube are greater than 2.1mb per second. It fails 100% of the time. You might get away with it for videos under 2 minutes but anything longer is an automatic fail. I also noticed that with the last update they removed the hashrate monitor and now you can't see. All the video hashrates are not equal even when downloading your video back from youtube. If you uploaded a higher resolution, the hashrate is greater even on the same 720p quality dl you get from Youtube. I've seen as high as 13MB hasrates for certain videos I used much higher output resolution when uploading to my YT channel.

So simple. You'll need to re encode your videos to a lower resolution or less date intensive codec to retain the higher resolution. So far I'm getting fails on anything over 2.0 hashrate. I am sure this is the problem. Good Luck and I hope it helps.