Windows 8…never had the best luck with that version of Windows. Especially if its 8.0 and not 8.1.
I would try on a different computer that has Windows 10. Also, a different usb cable and port.
Be sure u change the phone from Charging mode to transfer Files mode.
Also, I read about an update recently pushed out to some Moto phones that broke USB support, but I am pretty sure that is not for your model.
U could also install a File Manager app that allows u to acces the phones sd card via wifi. Asus File manager has this feature built in. Or, for some thing far more feature rich and powerful, AirDroid.
Though, I personally, just put whatever file I want to send to my phone on my Google Drive, then download the file via the Drive app on the phone. (u can use other services like DropBox, OneDrive, etc too.) Then u can use the built in file manager app to move the file from the Download folder to where ever u want on the phone it self.