CMMotionActivityManager.startActivityUpdates() is not working

I cannot get CMMotionActivityManager.startActivityUpdates() to work. I have given my app permission to use core motion and also put the required description key into my Info.plist file.

I am expecting my user to walk. I call startActivityUpdates() and then stand up and start walking. I typically get about 3 callbacks within a couple of seconds, all of which have "false" set for all of the activity flags, even "unknown". Then after that I get nothing.

I can't figure out what I could possibly be doing wrong here. The code is really simple. I looked at doezens of examples and my code is doing the exact same things.