I'm also having this issue
It only appears to work for a short period of time after it is booted up, even when using wifi to detect the position later. Once the GPS is lost, it's lost for good until reboot (bad to find that out on a roadtrip using the Nexus as a test device!).
Is this a defective unit issue or what?
Edit: It seems like if I open GPS Test up and let it run, it'll continue to track the position fine for several minutes (30+ minutes). As soon as I tab out of the program, the GPS ceases to function until reboot. Sounds like a software issue, but I'm wondering if everyone has it at the moment or just a few people.