This is VERY common.
Lots of people are complaining about it in other 'sync' topics in these community forums, and have been doing so for some time. Look around elsewhere.
Many people post solutions. Fitbit themselves ('Kate Fitbit' in particular) post solutions. Most solutions are tedious and time-consuming to do. I've been having trouble since acquiring my first Versa just over a year ago, and I've spent hours rebooting my phone, killing the app, reinstalling it, and doing all the other things suggested. Eventually, it always works again, and often for days or weeks. Then it disconnects and stays disconnected, till a reboot or two later it works again.
THIS IS NOT SATISFACTORY but Fitbit seem unable to fix it. My phone works fine with other bluetooth devices - particularly, it worked well with my Pebble watches.
Keep trying the various solutions. It will work. Then it will fail.
It does work better if you are happy with a daily data transfer and don't want the continuous connection you need for notifications, for 'Find my Phone' and similar facilities. I want these, so I have a clockface that shows the connection status. It can often stay connected continuously for a day or two, as it ought to do. But then the red bluetooth sign comes up again!
It is definitely a software problem in the Fitbit system, and IT NEEDS TO BE FIXED.
