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Mine is hit and miss too. If it's for a short sleep time, it doesn't add it. What I do is add it manually. That works every time.
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Hello @Tracylucas have a warm welcome to the Fitbit Community, it's great to have you on board. It's nice to see you around too @ACG. 
Please note that your Fitbit tracker starts monitoring sleep after 1 hour has passed and no movement have been detected. If your tracker didn't track the additional time of sleep after you woke up for any given day, is possible that it was unable to detect zero movement for 1 hour.
In any case, I would like to suggest you to edit your sleep to include the missing time by going to the Fitbit app and tap on the Sleep Tile, then on the Log you would like to Edit. Once in the log, tap on the ellipsis icon (...) on the top right corner of the screen and then on Edit Log.
I hope this resolves your inquiry, if there's anything else I can do for you, please feel free to reply.
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