Not logging all my progress
I'm doing Russian. When I finish a section, I make sure I do the strengthening exercises and it says I've finished that module. BUT, when I log back in, there are sections that say I haven't finished them. Why? I know I did.
Try refreshing your screen, especially if you are using the website. Also make sure you have a reasonably good Internet connection if using your phone or tablet.
I see the Duolingo website struggling a bit these days, possibly due to high load at certain times. That may be part of the problem as to why your progress isn't registering.
You can also try moving this post to the troubleshooting forum to see if you can get help there or search on there if others have reported the same issue.
Hope that helps.