It’s just like on Duolingo. When a new version of a tree is created, sections that you have completed previously will show incomplete. This is because new items have been added. In the same way, new items have been added to the Duolingo Memrise course and so sections where they have been added show incomplete, even though you may have completed that section previously.
I understand it’s a little disconcerting. I remember how I felt seeing my hard won Duolingo owls disappearing when a new version of the tree appeared (German, Spanish, Norwegian). But your progress has not been lost. It’s just an opportunity to learn something that was previously lacking. Usually you can quickly complete those new items. You may not have encountered it before in Memrise, but it is common on Duolingo. (And I’m hoping to encounter it again someday when wonderful Team Irish turns out Irish Tree 2.0. We’ll lose our owls temporarily, but we will get to learn new skills.)
The worst should now be over here on Memrise. I’ve been through all the sections of the course and made the major changes. Now I’m going back and adding sound files (when available). I’m also wiping out old phantom records from the database that pop up in multiple choice questions. The result will be a cleaner, more complete and hopefully more interesting and profitable learning experience.
Thanks very much for your patience.