Offline mode doesn't work on iPhone

I've been using Duolingo for two days. Today I wanted to learn again on my way to school but when I opened the application, it logged out automatically and it said I must log in. I tried to, but it said my account or the password was wrong. When I was online again, I could log in but when I was coming home and I was offline, it logged out again and I couldn't start a lesson because I couldn't log in. Of course I couldn't because there was no internet connection but I read that Duolingo supports offline learning too. What should I do? Is it normal? Thanks!