Unable to play video IPhone 6 Plus IOS 9.3.1

IPhone 6 Plus

IOS 9.3.1

Model MKUD2B/A

I have just run into an issue where I am not able to view imported GoPro video content on my phone. I connected the phone to my Windows 10 PC and was able to explore the folders on the phone and copy the MP4 files off to an external hard drive, I didn't cut and paste I just copied them. Now when I try to play the files on the phone I can see that the content is there but it but it just sits there with a small circle going round and round as can be seen below. I am able to play YouTube content without an issue.

I have tried restarting the phone but just not sure what to do now. I really don't want to loose the files I've imported from the GoPro as I've now deleted them from the SD card.

Thanks to anyone that has any suggestions on what to do next.