My Nexus 6 would connect fine via USB at work, but at home, it just won't show up at all on my Windows PC as MTP.
I realized that in Device Manager, it was showing up as an ASUS device, even though it is most definitely a Motorola Device.
At first I just uninstalled the driver, but it kept showing back up as an ASUS device in Device Manager.
Then, right I tried uninstalling it again, but I checked the box to delete the driver.
After that, I scanned for hardware changes. It took a while, but finally it installed a new driver on it's own, and now it just shows up as:
And it works.
I hope it helps you out!