Hello @goncalo.t3.
There are a lot of different reasons why a phone might not sync or work correctly with the Fitbit app. The hardware might not be the correct one for the job, there might be optimization needed because of the different software and hardware, or there might be a software on the phone that outright makes it impossible.
In this instance, it does seem like it is a matter of optimization given that it synced once. Some other users with similar phones might have found some workarounds or specific processes that can work so maybe they will mention them here.
Remember to check the compatibility list in case you need to know if a device is compatible, or what features does it support.
Let me know if you have any further questions.
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