Discount code can't be reapplied even if set to unlimited use

I continue to test the store I set up thru easy digital downloads.

When I use the same email to make a new purchase, and apply the discount code, a message comes out saying that that discount code has already been applied. I checked the settings for this discount code and it is set to 0 or unlimited maximum uses. What could be wrong?

https://www.ads-software.com/extend/plugins/easy-digital-downloads/