So for about 2 months now I've been putting up with talking into the mic during lessons and then sometimes Duo will just do nothing. I can listen to myself via the playback option but Duo just does nothing. So I lose a heart because I have to press 'I don't know'. I'm only writing this now because all too often it has occurred on the last question and yes of course I had no hearts left to spare.
That could be caused by a problem with your browser or your flash player. Try updating your flash player, and if that doesn't work, update your browser. If that still doesn't work, Duolingo might have a problem that they need to fix.