Make sure you're logged into itch.io with the same email you used to buy it. If it still doesn't show up, try using the "Recover Purchases" field on this itch support page: https://itch.io/support
Go to your itch.io e-mail section in your profile and ask for a verification e-mail to be sent to your e-mail address. Check your spam/trash folder in case it gets moved there. Verify your e-mail, make sure it is the same one you used to purchase the game on this site. Once verified you will be able to download the game. I just did this and it worked.
