Just thought I'd chime in, as I've had a similar problem as the OP and had a different reason/solution for it.
My GoPro 4 Black kept reverting back to 1080p/60 every time I tried to change the settings. Just ignored input from the Solo App. I even took the camera off and changed the settings manually, but they still reverted.
Apparently I had my GoPro in video+photo mode, and that was keeping me from being able to use any higher resolutions. I just had to change it back to video only, and it went back to working as normal.
Onezoso, I have a Hero 4, and that has happened to me a few times after stopping a recording. I've just exited back to the main menu of the Solo app and when I tap the Fly Solo button I get the video feed back, record capability returns. But... make sure you don't have the 3 Black's WiFi on, and I don't know how it behaves with the most recent firmware, but check your control settings to make sure they are consistent with what 3DR recommends (there is remote, app and remote+app for Hero 4).