When I picked up my model 3 today, initially voice recognition did nearly nothing. If I attempted to navigate by text, I got some odd notification akin to "offline mode".
Bottom line--my connectivity was not active. This seems really odd to me, but apparently Tesla voice recognition relies in considerable measure on real-time access to resources outside the car.
So if your voice recognition works sometimes, and not others, consider the possibility that it might be a connectivity problem. I think the base connectivity in the cars relies on one or more of the major cell-phone networks--and they all have some holes in their coverage.