Why we don't get a tier up? (Beginners question)

Hey guys, sorry for asking such stupid question here. I hope you can maybe help me out. Still pretty new to the hole Amazon Merch Game.

A friend and I started at the end of last year. To be honest, don't got any Sales until January. From this on, we found our niche. Even if it's only on German Marketplace (if this is the reason for not getting a tier up?) We sale around 30-40 Products a month (also only in Germany) All of our ten designs got at least one sale last year. And it's nice to see that people buying the designs.

I know that there is no official guideline, but from what I saw only we should already get a tier up. Maybe you can tell me what we are doing wrong? Thanks :)