That's strange, you should be able to download newer builds if you bought the game back then. It was priced at $10, which is the current price point for the Build#2 releases... The only way you'd be locked out of new releases is if you'd bought only Build#1 for $5, which is basically just the demo version. But I've also heard that Itch can be finicky at times with separate files/pricings, so it could be just that...
Shoot us an e-mail with your purchase invoice attached, and I'll hook you up with an Itch game key that'll give you access to the latest versions! :)
~ Frenzin