SEO - intermittent pages
Wondering if you can help me, we have recently moved from an old HTML site to a WordPress site and created all of the correct 301 redirect, landing pages etc. We expected our rankings to drop around 10-20% which they did. Now, around 1-2 months into it some of the pages which were showing on Google a few days ago have completely dropped, however they have reappeared on a different pages of Google and then randomly reappears on the first page again.
The sitemap has every page included so there is no reason that on the backend side why this would happen, my concern is where specific pages have completely dropped off, I can confirm this by typing in "site:websitename.com" and I do not see the page I knew was there before like a few days back.
There are no errors in Google search console and all looks healthy, so I'm looking for some advice to see if there is anything I am missing?