DIAGNOSIS Sleep Sensing requires very specific conditions. The most common reason it fails is a change in your room's environment or the device's position that breaks the motion and sound detection.
FIX Walk through the setup checklist again. First, ensure the Hub is on your nightstand, about arm's length from your head (roughly 1-2 feet), with a clear view of your upper body. The room needs to be dark at night—even a bright LED from a charger can interfere. Check that Sleep Sensing is still turned on in the device's Settings under Sleep Sensing. Most importantly, verify that Microphone is on (the hardware switch on the back is not flipped to off) and that Motion Sense is enabled in Settings > Audio & Video.
IF THAT DIDN'T WORK The calibration might have been lost. In the Fitbit app on your phone (where sleep data appears), find your Nest Hub device settings and look for an option to recalibrate or retune Sleep Sensing. Follow the on-screen instructions. Also, ensure your Google account used on the Hub has not changed primary accounts, as this can break the data link.
HEADS UP Sleep Sensing won't work reliably if you have a partner, pet, or fan creating significant motion in the detection zone. It's designed for a single, relatively still sleeper in a quiet, dark room.