01-18-2016 18:34
01-18-2016 18:34
01-19-2016 00:59
01-19-2016 00:59
This can be down to a number of reasons. The usual culprit is that only steps directly tracked by your fitbit count towards challenges. Any steps from manually logging an activity, from a 3rd party app or from tracking via your phone's GPS don't count.
Another cause of issues can be where you are in a different time zone from others in the challenge. There is more information on this at:
http://help.fitbit.com/articles/en_US/Help_article/Challenges-FAQs/