I think browser downloads can sometimes fail. I don't deal with that, because that is the Itch platform, that I have no control. However, I do believe if you use their free client for the download, it does not have the problems that browsers sometimes have. You don't have to use it to launch the game, but their client my provide a better download experience. Try that. If it doesn't download everything properly, you may have to raise a support issue with Itch.