Nexus 9, Lolipop 5.1.1 Win 8.1 - why won't my device appear in MTP mode?

[AC Question]

Nexus 9, Lolipop 5.1.1 Win 8.1 - devise will not appear in MTP mode

I have a brand mew Nexus 9 LTE with Lolipop 5.1.1. When I connect to my Win 8.1 computer in PTP mode the Nexus shows up. But when I connect in MTP mode - nada, nothing.
In PTP mode the Nexus appears in the Device manager but when I move to MTP, is dissapears.

HELP!!!!!!!