It looks like NVIDIA has put a temporary stop on the Marshmallow update for the original Shield Tablet. A NVIDIA representative has posted some troubleshooting tips on the forums in case you've been experiencing any Wi-Fi problems.
Operation Flashpoint: Dragon Rising comes to NVIDIA Shield
Although this update won’t exactly revolutionize your Android TV experience, it will bring along some nice perks that can make interacting with the device simpler, more intuitive, and more customizable.
12 Days of SHIELDsmas brings awesome deals on games for NVIDIA SHIELD owners