I was wondering if im missing something? I added the 'Are you my Mummy?' plushies and 'Adrift' plushies to my collection but im not getting the achievement when viewing my collection

Go into your achievements page and find the achievements you want to activate. You have to click "Show Required Items" and if all are showing as green and checked off, it should award you the achievements.

Quick links: https://subeta.net/achievements.php#/achievement/881 https://subeta.net/achievements.php#/achievement/1367

oh now its working, could it be that the page needed some time to update?

Yes, sometimes it takes a little while to update. Especially if you've viewed it recently - like checking which items you still need.