I've never experienced the same outcoming after a migration; each site is different and also are the organic competitors.
It's common to lose some traffic just after the change but if you made a good migration and the new site is better than the previous one, you will recover and grow faster in about 4/6 weeks (based on my experience).
It's difficult to get a straight answer without checking the site but in order to help I'll give you the steps I'll follow to make a successful migration:
Check robots.txt for disallowing instructions
Check nofollow tags in the header
Check analytics codes
Update internal links with the new URLs
Check canonical tag
Check duplicate content
Check 301 redirect from old deleted URLs
Check www and https redirect
Create and submit new sitemap (Search Console)
Check traffic, performance, and rankings (Analytics & Search Console)