Promo code not working, but URL does

Our app has been live for a while now and is working well. However, our promo codes have never worked correctly. When a user tries to download the app and use the promo code to get 6 months free (for example), it doesn't work. But when that same user uses the URL link provided to get the promo, that works just fine. What are we doing wrong?

Promo code not working, but URL does
 
 
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