Most times when I turn on my Phantom 3 4K the light on the front of the gimbal mount flashes green continuously and there is no video feed on the DJI Go app.

I've found that if I select auto-calibrate gimbal in DJI Go this sometimes turns on the video feed.  Also, altering the gimbal tilt in DJI Go sometimes restores the video feed.

I wonder if anybody else has experienced this and knows the cause.

Hi thanks for reaching DJI Forum. We do apologize for the troubles you had recently with your DJI Phantom 3 4k. I understand that when you auto-calibrate your gimbal using your app sometimes it turns ON the video feed, Id like to ask what is the brand or make of the mobile device/tablet you used to download the DJI Go app? With that being said, I will be providing also some of the fully compatible devices our engineers have tested. Should you have any concerns, don't hesitate to reach us. We're here to help you. Thank you.

I use a Lenovo Tab 4 8 with Android version 7.1.1

The same thing happens with my phone - a Huawei Mate 10.

Neither device has any difficulty reporting all other aspects of the connection