If you use it from some time and made just the upgrade from the pilot app to the go app, you can try to uninstall the app, delete all DJI folders before reinstalling the app. Don't forget to upload to DJI server your flight logs before so they can be restored.
If I don't get the connection at start (very rare since last update), I exit the camera view to go back to home and I try to disconnect and reconnect the USB cable. If the button "Enter Camera View" is blue, everything will go nicely. If it is grey, it will not work.
Also, check the "Developper option" to see if "USB debugging" is enabled.