I don't know if there is an actual trick to it as (touch wood) I haven't been having many problems syncing... Sometimes it can go a bit funny after the App has had an update. It might be good to check that everything is up to date first.
Maybe turning on Bluetooth before opening the app on your phone might help to get a quicker connection. If mine isn't syncing fast enough I pull down from the top of the screen to 'refresh' and trigger it to work.
Someone else might have an official trick for you.
I got my Charge 5 on Sunday and had the same issue (Android). My Fitbit is paired but doesn't show as synced. After wasting time troubleshooting I called support and was told that this is now by design.
Fitbit no longer syncs automatically, you have to manually resync your Fitbit via the app by pressing and scrolling down from the Today page. You can also do it via the app from your account settings but it's extra step.
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