I have a hebrew website (guyariv.com) dedicated to public speaking, that was recently migrated to wordpress from drupal. It is running a fairly complicated theme called EDUMA which is a learning management system with lots of features and plugins.
The old drupal site was very outdated (not even mobile responsive) but it had 2 things going for it: it was very fast and had absolutely terrific SEO. It has loads of quality original content and a fair number of incoming links.
Since migrating to wordpress my search ranking dropped – I used to have first page ranking, sometimes for more than one page. Now my first mention is on the 2nd or even 3rd page and that's for the main keywords. In the long tail it's even worse..
I was told things like that happen during migration and sometimes are temporary, but I want a pro to look at it and see what can be done.