IPhone camera roll won't play videos?

Someone please help me.
I have an iPhone 5S and whilst scrolling through my camera roll recently, I discovered that some of the videos will not play - but instead come up with a 'buffering wheel'? There's no bar at the top which shows the content of the video either, but the thumbnail still remains. I'm so upset about this if it means I've lost the videos, as they are extremely important moments!

P.S the videos in question are all ones SAVED to my camera roll from snapchat, rather than being taken on the camera mode.

I'm having the same problem on my iphone 6! I don't know why this is happening. Please post on here if you find a solution!

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