Could be the GoPro is freezing ... have had this issue a few times since owning my solo, GoPro will freeze, and even though you may still see a live view from the GoPro in the Solo/Solex app, the GoPro cannot be controlled either thru the Solo/Solex app nor can the GoPro be controlled by the GoPro mode/setting buttons on the GoPro itself. When this has happened to me , it does appear as all control of the GoPro has ceased to function. When this happens, I have pulled the GoPro form the Solo Gimbal and wasn't even able to power off the GoPro using the GoPro power button ... had to pull the battery from the GoPro to power / reset the GoPro. GoPro powers back on after doing this and works fine, and can be controlled via the Solo/Solex app. I have read from various sources that the primary cause of GoPro freezing could be a bad SD card. Try formatting the SD card (on PC, not using the GoPro itself) and see if that helps.
Also, I have read that using any GoPro firmware version newer than v3.0 on the GoPro, may result in control/compatibility issues between the Solo Gimbal and GoPro.