I have had same problems with my iPhones (5s and 7). I found that my problem was my phone was just too full, and it was causing phone to act "buggy" and slow. I was able to transfer small pics but videos were a nightmare, and often the iPhone would become unrecognized after one video transfer, or even before. Then I would have to unplug/replug my phone in to pc and try again.
I found that a newer lightning cable (the one with my 7 not my 5s) helped a little bit.
But what really made the difference for me was clearing off several GB of space on my phone (delete or offload some apps and stuff like that, which was also very difficult to do while phone memory was low).
Once I had more free space on my phone than the size of the largest video I had, then photo/video transfer from iPhone to Windows pc was a piece of cake again.
I guess just like with Windows PC's, you should always try to keep a good chunk of free space for it to work with. I wish I could set a warning on my iPhone, when I only have a couple GB of free space left, so I can make it a priority to move/backup my pics BEFORE phone acts flaky.
And I really wish I could make the iphone pause taking video (or at least give me a warning), before it records onto all the free space left on my phone! It has happened many times as I was recording, that the video stopped, but only because there was no space left on the phone. That seems like it would be great, except that then the phone is slow and buggy and it's really hard to transfer pics/videos off to regain that free space needed!