I waited a month for more credits

Your credits are refreshed monthly based on the date you started using Firefly not at the beginning of each month. So, for example, if you started using Firefly on Nov. 24, your credits will be refreshed on Dec. 24. 

Here is more information about how Genreative Credits work: https://helpx.adobe.com/firefly/using/generative-credits-faq.html

Hope this helps.