Here's what I do and it works for pairing and syncing. I figured it out after I accidentally upaired, and couldn't get it to work.
1) Go to settings, then apps, then find the app and force stop it. Works on any phone that supports the device.
2) Turn off wifi and reconnect (make sure you have a solid connection).
3) Turn off bluetooth off then back on.
4) Reset your fitbit. This must be done while it's on the charger and at least about 70% charged. Reset by holding down button (while on charger) for about 10 seconds until you see a smiley face. Sometimes legal info will pop up, too. Leave band on charger when done.
4) Go to bluetooth and try to pair. Stand as close to your band as possible. If this doesn't work go to next step.
5) Open Fitbit app, and click on the icon on the top left corner. Under "devices" click on your device then click "pair."
6) Go to your bluetooth and verify it's paired.
7) If none of this works restart your phone and start process from step 2. Sometimes is can take a few tries, but it works.
