Videos won't transfer from iphone to PC

I am unable to transfer any of my videos from my phone to my PC. Photos are fine but when I try to transfer videos, I get a message saying "A device attached to the system is not functioning" even though the phone is working fine (as I said, photos are transferring fine). If I try again, I get the message "the device is unreachable" even though it is still connected and working fine. I have tried 2 different laptops and the same thing happens. Nothing has been changed on either the phone or the laptop so I have no idea why this has just started happening. If I ever do manage to get a video across to the laptop, when I try and play it I get a message saying "This file isn't playable..." Can anyone help? Many thanks.