Why does my Nexus 7 (1st gen) never let me copy my files to my PC?
I was having a similar problem.  What I did was install ES Explorer on my Android and use the LAN option.  On the PC I setup a share folder(s). In ES Explorer I went to the folder with the file(s) I wanted to transfer, I copied them. From there I went to LAN page and tapped on search, takes a few seconds, then tapped on icon for my PC (192.168.x.x.x). Then choose where I want to put the file. Real easy work around

You can also use ES Explorer to transfer between other devices (wifi hotspot, Bluetooth, FTP...)

Good luck!!!