To quote a different reply I made: "...if you do buy here rather than the Patreon, always keep in mind how itch calculates your purchase: The money you spent is your tier. As in if you buy a game 50% off for $5, you can't download the $10 normal price one when it's no longer on sale. Weird I know, but that's how this site works."
If that's the reason you can't download, then it's Itch doing it.