As for the download to device and upload to cloud post-flight procedures, they might fail time to time. We believe that this is not a systematic problem since it is not possible to reproduce it. It might be unstable in some cases. Once it fails, it happens that it cannot be initiated again and become successful.
In case, if you use the X5 camera (I1, M100, M600) or Mavic Pro, the automatic synchronization is expected to fail (even when triggered manually). It is a known bug that cannot be fixed at the moment because the DJI SDK (programming tools that allow the communication between the drone and an app) does not support the synchronization.
In case the synchronization fails, there is for now no other option but to extract the images from the SD card of the drone and create a new project from scratch, for more information: 204911905.
For more information on synchronization to the cloud: 204716225.
Hope this helps,