This seems to be a common issue with these considering when I was having this problem I found a bunch of posts on this, but after the trouble shooting options are done and you still have this problem there solution is just buy a new one here is a discount code which is what just happened to me. This is my 2nd fitbit item and I am questioning whether I want to continue to buy from them because they are having known issues with some of these trackers that are not cheap and their only solution is trying to get you to buy another one that will probably brake in 1.5 to 2 years if not less, and because its out of the warranty time frame they just say they can't replace it. Very frustrating when one week it was staying charged for days and then all of a sudden out of no where it won't even stay charged for a full day.
