If APP can't find any data from your device,pls try the article to fix the problem:

1. Restart your device and upload again , in case the activities hadn't been successfully saved.
2. Delete some of the history fit files from your device.

   The latest activity will be not be found if there are too many files inside the device, as APP can only recognize limited files.

Steps to delete fit files :

1) Power off the device, connect it to computer.
2) Short press left button,the device will be recognized as a new disk.
3) Choose and delete the fit file in the activities file.

If it still can't be transfered, pls try to provide us the exact fit file for further analysis.

You may also turn on the "auto deletion" from APP to delete the data when it is successfully uploaded. 

More article,  refer to APP auto deletion