A Modest Proposal
It is my conviction that if you get a question wrong, but then your reported translation is accepted as correct, then you should have that heart returned to you. Each user of duolingo should have a small bank of extra hearts earned from accepted answers that can be used if one runs out of hearts. If the program is keeping track of "lingots" then surely it can also keep track of extra hearts that can be used later on. It's only fair :)
The main reason I never report problem is because this feature is not available on the apps. There have been many occasions when i have spotted an error and know it will go unchanged because I can't report it. I'm not too concerned about losing hearts but i don't like the idea of myself and others learning incorrect translations so i am really looking forward to them adding this feature to the apps.
I'm really hoping they will add this. I use the app exclusively, when I am not fishing through the immersion section, since it means I can get a break outside. The other day a sentence was translated from 'Krankenhaus' to just 'House', and there have been numerous other occasions that I would have liked to provide feedback.
cloclo15 and chilvence are right, but a lot of people using duolingo on a pc are still failing to use the button and complain only in "discussions" (without bothering to use the button)