Have you exceeded the 32 ZigBee device limit in SmartThings? I had a similar problem also with an Aqara motion sensor. I ended up adding a xbee SC2 device to my network to act as a router and that has solved the problem for me. I now have 45 ZigBee devices, including 11 Aqara motion sensors all working on my network.
Refer to this thread for more information: Zigbee - from flakey to stable