It is a little soon to be certain, but in the course of troubleshooting, I sent them photos of the watch on my wrist (to prove it wasn't too loose), and the Apple advisor commented that I was wearing it up on the wrist bone area, so the underside of the watch wasn't making solid contact. I do vary the positioning throughout the day, between up on the wrist bone, vs. moved about 1/2" back in the direction away from the bone, when I feel like it gets uncomfortable in one position, I do the same with rings. However, it seemed kind of obvious when he said that "on the bone" wasn't giving full solid skin contact, so from that day I started making sure it was positioned about 1/2" back away from the hand and wrist bone right before I go to sleep, so it was in the softer non-bony area of the forearm, and for the last five days in a row I got a wrist temp reading each night. I'm kind of smacking my forehead right now that I couldn't have figured this out myself. On the one hand, the watch had no problem giving me sleep patterns every night regardless of how I positioned the watch, on or off the wrist bone, but the underside of the watch apparently needs to be fully flush on the soft skin area to get the temperature (which makes sense when you think about it).