Just joined the windows insider program (fast ring)--Lumia 640. I thought I would get build 10512, but I got build 10166. Any reason why the Lumia 640 would not get the latest released build?
Go back to application Windows Insider and go back through the steps, making sure you're on Fast Ring. Build 10166 is a Slow Ring build now so you probably accidentally got onto the Slow Ring. No big deal just go back through and you'll get Build 10512.
I'm the OP--I was sure I had selected the fast ring, but I also noticed that the 10166 install had other weirdness-like no web browser installed. So I did a hard reset and got a clean 10166 install, then added the insider app, selected fast ring, and voila, it is now updating to 10512.