IPhone app does not allow progress to following lesson
I have an iPhone 4, and have duo lingo installed. It's been working fine till today, when I successfully completed German lesson 4, and didn't let me progress to lesson 5, which remained locked.
I the accessed and completed lesson 4 in my browser, ans was able to access lesson 5. I went back to my iPhone, and lesson 5 was available. Again I completed it with one, two and three hearts, and it would not let me go into lesson 6.
Is this a bug, or what?