For undisclosed reasons T-Mobile has temporarily halted the OTA Marshmallow roll-out on the 2013 Nexus 7 LTE. The OTA software update originally started on October 5th.
We can assume the reason behind the pause is bug-related. If you have been patiently waiting for the update, never fear, it looks like roll-out will resume again sometime in early December. We'll be sure to share any new information with you as we receive it.