I complete a section in my level and days later, it appears uncompleted. Why?
Does it appear uncompleted, or just change from gold to colorful?
For example, if there are 8 lessons in the unit, does it suddenly say 7/8?
Or does the icon turn colorful and if you click on the unit do all the lessons inside still say "Redo", indicating you've completed them before?
If it's just turning colorful, then it means that you need to review that unit. Language acquisition (and any learning, for that matter) requires review.