There are multiple factors taken into consideration while calculating DA. In this Moz article, DA is explained by Moz itself.
In short, possible reasons could be no backlinks have increased, referring domains are the same regardless of backlinks quantity, and website's credibility.
I faced the same problem for my SEO agency around two years ago. Then, I shifted my focus from the quantity of backlinks to quality and local backlinks from and around Sydney. And guess what? It has been significantly improving and it has also impacted organic traffic positively.
