Why won't songs that play on my iMac, play on my iPod?

They're 5th Gen iPods, pre-touch models. Not sure what software version is on them, but every time I sync them my Mac says it checked for updates, so it's probably reached the limit of which OS can work in that model. It began happening, probably in the last year or two, likely due to the OS on my Mac, being updated, and it affecting how music files are read. I think it has something to do with something called VBR, as the iPods will play files @ 256 kbps if it also shows it as (VBR), while otherwise it converts 256 kbps files to 128 kbps.

This wouldn't even really be an issue, if the Sound Control (normalization) feature on my new iPad Pro 12.9 actually worked. I can't rely on using it to play tracks on my gigs, as some songs end up blasting through the PA, which is not cool. Wonder why that same feature works fine on my ancient iPods, but not the state-of-the-art iPad Pro. And no one at Apple appears to have a clue as to how it can be fixed... which I find incredible.

Not sure how it would be related to storage, why would anything related to storage determine what files can play? It's pretty clear it's got to do with "VBR" for even higher bit rate files will play if they are "VBR", while converting higher rate files adds a lower bit VBR file So I am pretty much forced to have two files in my Music app, of every song that otherwise wouldn't be played on the iPods.

As for the Sound Check, I've already spoken with a number of Apple support people, "geniuses", store manager, and an engineer who said he'd look into it and get back with me, which he didn't.... IMHO, if I buy a device that has something on it that doesn't work, & they can't fix it, I should get a replacement. That's how most companies do business...