Motion tracking no longer working on 6S

I recently purchased an iPhone 6S, and the motion tracking worked for a few days, but then in the past four days it has recorded no steps or flights climbed. This morning, the phone started recording data again, but only briefly (maybe 2 hours); it currently is not collecting any motion data. I've been using Apple's Health app and the FitBit app. I tried turning motion tracking off and then back on, restarted the phone, no luck. I'm on iOS 9.0.2. Any ideas?