Generally there are few reasons why your subscription key wouldn't work. But realistically, sometimes technology and credit cards go awry.
If you purchased a subscription, failure of subscription key will happen if there was a technological error, the payment from that particular card failed, or there was a CVC check and nothing went through.
When either of those occur, we try for three (3) more days and if nothing changes or goes through, then we completely revoke the license until you update it. Stripe will send you an email confirming that fact and informing you that no payment was actually charged.
From there, you'll need to try to re-purchase.
Everything was okay on the purchase side but your subscription still isn't working? Send an email our way at firstname.lastname@example.org