I had a problem with my Fitbit Charge 3 frequently refusing to sync, meaning that when I noticed I had to reset it causing it to lose days worth of data.
When I contacted support I was told it was because the Motorola G7+ (Android) phone I was using wasn't officially supported. If you look at the list of supported devices it's actually very small compared to the number of devices on the market:
I eventually found out that the solution was to delete the Fitbit from my remembered bluetooth devices, after that it would sync.
Fortunately the latest update has fixed the problem for me ( the opposite experience of many, it seems ).