There could be more than one reason. Sensors don’t ask for much; all they want is a good spot and proper power. However, sometimes -even when fully charged- they continue to disconnect. This could be because they’d like to be closer to The Hub. If it seems as though there’s too much distance between these two, try repositioning the sensors.
Here’s some info: 'How do I check if my sensor batteries are low?' on battery levels that you should check out. If power’s looking good, then we’ll want to try repositioning the sensors here too.
If your Hub is working just fine in the place you gave it, then ideally, it’s the sensors you should reposition to be more central to The Hub. But if a Sensor -say, a Door Detector- cannot be moved, then you might want to try a different location for The Hub. That way, all sensors can have good communication.
If your sensor continues to disconnect, even after all the rearranging and repositioning, contact Customer Care. They’re really good at this!