Read: Supported Zigbee adapter
" Previously the Texas Instruments CC2531 was the most recommended adapter. This adapter is not recommended anymore since it’s not very powerfull, doesn’t have good range and struggles with larger network (20+ devices). It also requires additional hardware to flash the firmware, see [Flashing the CC2531].", replace it with recommended adapter.
I have same problem it’s not stable for couple months already, But was good for couple years. I did order new adapter,