I think it also has to do with keywords. A lot of my videos are gun related. Earlier this year, suddenly, for the life of me my videos wouldn't monetize anymore. So, for my upload files, instead of "AK47 Armor Piercing Steel Penetration test.MP4" I named them things like "video.mp4" with no title or description. It monetized within minutes, I then renamed it and edited the description.
Now I upload as private with generic names and don't put any description or keywords in until it's monetized. Recently I mistakenly didn't rename a video and I put the description and keywords in during upload, it took hours to monetize. My generic uploads take minutes.