The Fitbit app on certain phones with certain devices can sometimes fail to sync. This might happen multiple times a month. To workaround this issue, try force stopping the Fitbit app. This normally shouldn’t lose any data, and reopening the Fitbit app and dragging down should re-sync in a few seconds.
Also, check that Bluetooth is enabled on the Android device.
Finally, try rebooting the phone and watch if the previous steps don’t help.
Force stop an Android app by:
Settings → Apps and Notification → Sell all apps → scroll to app name and click → click Force Stop