'Create iPod or iPhone Version' only working for some video files, what gives?

I've been falling asleep watching/listening to various YouTube videos for some time now. I tend to play them via the app on my PlayStation 4. I decided to transfer some to my iPod Classic (160GB) for when I'm away from home and/or a reliable internet connection. My iPod software is up-to-date (2.0.4), as is my iTunes (12.7.1.14), I am running Windows 10 on my laptop.

I download the relevant videos using JDownloader 2, at the highest quality settings that program allows. Obviously this is too high quality for my steam-driven museum piece of an iPod, so I used iTunes' built-in conversion option to make a copy more palatable to my outdated hardware. This worked well for one series of videos, making me confident I had cracked the code. Unfortunately not, as we shall see:

The first file (from left to right in the image) was from a series of 8 videos I added successfully using the Download-Convert-Sync method. All work perfectly on my iPod Classic. The second file was treated the exact same way and adamantly refuses to sync to my iPod. Plays fine in iTunes, but I keep getting an error message saying it's not the right format. I put the specs of the converted files side-by-side to see any discrepancies, and the only one I could conceivably see being a problem was the dimensions. So I re-downloaded the second YouTube video, with the same settings except for a 480x360 dimension rather than 640x480. Again, no luck.

To reiterate, BOTH OF THESE FILES HAVE BEEN CONVERTED WITHIN ITUNES TO BE COMPATIBLE WITH THE IPOD CLASSIC. This is especially infuriating when every forum online suggests converting via iTunes is the fix-all solution when it clearly is not. I am baffled as to why it works for one file but not the other, when they have undergone the exact same process. My frustration has reached new heights, in that I have created a forum account specifically to ask this question.

If anyone has any insight to offer, I will be most grateful,

Charlie