Many people will give you the old "please try these steps" spiel. I have found none of them to be the consistent answer. For quite some time, I just accepted that the band wasn't tracking sleep because of the fit, or because I moved too much in my sleep, or it lost the heartbeat. All of which would make basic sleep come up instead. Plus for me, it never syncs sleep unless I add it manually, which is another problem altogether.
Here is what I do instead, and I get sleep stages every day. I start the app, then turn airplane mode on for the phone. I wait for the app to say it is disconnected, then turn airplane mode off. I add sleep, tell it how long I slept, and like magic, sleep stages appear. If, in the rare event, it tracks partial sleep, I delete that one, airplane on/off, then add the correct sleep time. I have even been able to go back a day or three and delete/re-add prior days and they change to sleep stages.
As I have been able to get this to work consistently, I conclude the problem is with the Fitbit app. I hope this gives you actual help without having to go thru all the hoops that never worked for me.
Get your app fixed Fitbit!
