I've run into an odd issue; I recently purchased an original GoPro, that has the Micro USB out only. I wanted to play back some of the videos on my Z30, so needed an adapter cable to go from Micro USB to the mini HDMI. These cables are referred to as "Micro USB OTG"
I have hooked up the camera to my phone, and it works the first time. Phone displays USB device inserted, then blinks Removed, then says Inserted. I then can watch my videos, but if I take it out, and try again, the phone never senses that it is connected again? If I take out a new cable (trying this in the store), it goes through the same process, but will never allow the same cable to be plugged in and sense the device. It's almost like every individual cable has its own IP, and it won't allow it to connect again.
GoPro only offers download to a device that can automatically store the video. they offer a program called Quik for desktops and Android / IOS phones . I would suspect the video will download possibly only once per connect. The instructions state to connect the device then power up the GoPro for download. If the download works with the USB cable you have once then it should be ok. Try doing a repower as they suggest per download.
Also mentioned is a need to update to the latest software version on the GoPro.