Failed to enter upgrade mode P4

I downgraded phantom 4 std with NDL, but now I can't upgrade anymore. From the following message: "Failed to enter upgrade mode" with 0%. What should I do?

Drone Model:  Phanton 4 std
Current Drone Firmware:  01.02.0602

Sorry translate

This may seem counter-intuitive, but try either reformatting the SD card in the P4 **or** replacing the SD card with a new one.

Please let all of us know whether this either helped or did not help.

I have the same problem with my phantom 4 and the sd card format does not help

I have a P4 std and downgraded to Vers 01.02.0602 and get the error message "failed to enter upgrade mode"  As far as I can see, the drone can arm, and seems to fly, but I cannot upgrade or downgrade the SW. Reformatting and replacing the SD card didn't help. What else can I try ? Will I be able to reset to factory settings through DJI App ?

OK, I fixed the drone by starting with the normal DJI Assistant 2,connected the drone and switched on, did a factory reset in the app and then tried to upgrade to the latest SW...it came up with the expected "failed to enter upgrade mode"
I then closed the DJI App - with the drone still connected and switched on - and opened the NLD App, did a SW re-flash to the existing SW version and viola, it worked. From there on I could upgrade to version .700 and did all the batteries and the NFZ patch. All upgraded 100%.