So far we can see one main reason. Sometimes there are some delays when sending the mission details to the drone through DJI SDK (what allows the communication between the two). A time-out could prevent the mission from being completely uploaded. We would suggest to try the following:
1.a. Click the Abort button to interrupt the mission or take over manual control to land the drone.
1.b. (alternatively) Cancel the mission meaning click Cancel in the takeoff checklist pop-up if the mission upload failed.
2. Restart the mission but slightly adjust the grid before, e.g. the size, in order to force the app to upload the mission details again.
3. (optional) If this does not work, restart the mission scratch, meaning close and force the app to stop, disconnect the mobile device, switch off the remote controller and the drone.
There are differences because the SDK is specific to each drone and to a platform, meaning that the Android SDK is not the same as the iOS SDK. Also from one mission to the other, many factors can affect the continuity of a flight this is why the problem can happen time to time. If you fly a new mission on Android, the problem might not arise.
In the meantime, I have sent you an email to test our latest app that is beta if you wish.