Lots of problems syncing with Android app and any devices which are not on the compatibility list.
The Samsung A5 is not on the list, this doesn't mean it won't work it just means not tested and no changes to Fitbit devices/app to support device.
Some things you can try here:
http://help.fitbit.com/articles/en_US/Help_article/1866/?l=en_US&c=Topics%3ASyncing&fs=Search&pn=1
Keep an eye on this page as new Android devices are added as they are tested.
https://www.fitbit.com/au/devices
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