Recently we got a new feature in the Incubator that enabled us to make the nuqtas for क/ख/ग/ज/फ optional and to show a typo message if a user missed the nuqta for ड/ढ (as they are mandatory in the case of these two letters). But there seems to be a bug in the system, so that the user is being marked wrong if he/she misses any nuqta. We've reported this to the Duolingo team, and they are doing their best to fix the problem. Please bear with us till then. :)
You can check this out : http://www.learning-hindi.com/post/1187496443/lesson-54-dotted-consonants.