Cold can affect battery efficiency, battery efficiency affects transmission range, it could be something as simple as that. 
Often the process of replacing a battery or even just resetting one warms everything up enough to get things going again.
The strength of a mesh isn’t based on the total number of devices – – it’s based on the total number of repeaters and their position. Ideally you’ll have one repeater about every 40 feet for zigbee. If there are architectural barriers you may have to be more precise with placement. For example, in a cement house of the type often built in Asia or an adobe house like those in the American Southwest, you may need a repeater near every doorway so you can get signal to go around the walls since it can’t get through. Once the weather gets cold, you may need repeaters a little closer to the sensors. 
As was suggested, Taking the hub off power for at least 15 minutes to force A network heal is always a good idea. A lot of commercial installations will do this once a month, or even once a week just as a maintenance step.
@johnconstantelo has some wellness check code which looks at the event logs to see how long it has been since a device checked in. That’s clever, because it doesn’t use up battery on the sensors since it doesn’t talk to them directly. After he shared it, several other community members worked on smart apps to check individual devices. This thread is worth looking at.