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I note that the J3 isn't on the list of supported devices at www.fitbit.com/devices so there may not be a long-term fix for this. However, you could try restarting your phone and then restarting your fitbit using this procedure:
Give this a few tries as it doesn't always work. You might also try forgetting the fitbit in your phone's bluetooth settings. If it still won't sync after that then there are some more ideas to try in this help file: https://help.fitbit.com/articles/en_US/Help_article/1866/