Had one of their motions that stopped working for me as well. Gonna go Xiaomi route next.
Ran into the same issue when installing three door sensors (2 of the 3 work without incident), the third:
- Paired with smart things hub
- Properly indicated ‘open’ immediately after pairing
- Properly indicated ‘closed’ when closing for the first time (although with ~5s of lag)
- The device stops sending commands when contact is moved
- Reset a half dozen times, let it think about its life-decisions overnight, etc., with the same results
If 2 out of 3 are working fine then it sounds like a defective device and you should return it.