Could be a failure of the Zigbee radio itself, but it could also be that you’ve hit one of the maximums. There is some documentation on this, but it’s very deep in the developer information and it’s hard to find.
So… how many total devices do you have (counting all protocols, including virtual devices)? There is a limit of 200, and after that you aren’t allowed to add more. 
How many Zigbee devices do you have which are not repeaters? This includes all sengled brand devices as well as most battery powered devices. It does not include Zigbee devices connected to a Hue bridge.