Hello @sarahmatthews88, let me give you a warm welcome to the Fitbit Community! When this happen it can be an issue with your timezone configuration. When the steps are reset to zero this is control by the your local timezone. If this is incorrect on your account settings it may caused your steps go back to zero in the middle of the day.
If this is the reason of the issue and still persists, please review your timezone settings. For more details take a look at this post.
Hope this helps and let me know how it goes.
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