Why do some of my pictures not show up when I plug my iphone 5 into my PC?

I have an iphone 5. I regularly transfer photos from my phone to my PC by connecting it with a USB. For some reason, some of the photos are not appearing in the internal storage folder. Yes, I have checked all the folders. When I first plugged it in, my phone had over 1,200 photos, but I was only able to transfer and delete ~350. The other 931 do not show up. Any ideas? I already tried unplugging it and making sure the phone was unlocked before plugging it in. This has never happened to me before and I do this every other month.

Thanks!