My main YT account shows ads just fine.
I created a secondary YT account and linked it to my Adsense.
Everything is fine. Everything is monetized. Everything is public.
But ads don't show, period. Ever. If I disable and re-enable monetization, it does nothing to fix it.
I've tried everything short of deleting all videos and reuploading them (but new videos I upload still don't show ads, so I doubt this is the issue).
Pretty frustrating, actually.