You can read more about Moz's Domain Authority here. There are lots of factors that go into it. It may be a matter of time, it could also be a number of other factors. https://moz.com/learn/seo/domain-authority
Ultimately, though, whatever your DA score is doesn't matter so much...it is an indicator of success, not something Google or Bing uses. Even Moz says in that article "The best way to influence this metric is to improve your overall SEO."
Given that, I'm more curious how your domain is performing in Google or Bing following the move. You mentioned rankings are holding, but what about traffic from search and the quality of that traffic? Did any rankings increase or decrease? What changes can you make to increase rankings? What can you do to improve decreased rankings?
As well, what do Bing Webmaster Tools or Google Search Console have to say - are there any new errors or issues, what does search analytics show, what about the count of indexed pages? (For more on Google Search Console and what to check out, you may find this video helpful - https://www.youtube.com/watch?v=f7iSlnD6sEM)
Basically, my point is that Domain Authority is something interesting and helpful to look at to give you an idea of overall comparison to other domains. I use it often with my clients, especially related to link building. But, Moz's DA isn't something I'd really worry about or focus on improving. It really isn't the sign that your site move worked or not. If rankings, traffic, traffic quality, errors, indexed pages, etc. are all doing well (or improving) following the move, then that will tell you if the domain move worked or not.