I read that this is a known issue with earlier releases of the device.
Sonoff web site says:
If your dongle’s descriptor hasn’t been customed, you can use the tool linked below to change it to “Sonoff Zigbee 3.0 USB Dongle Plus”.
If your dongle still can’t be auto-discovered, it may be because your version of Hass hasn’t integrated this feature yet, you can follow the normal configuration steps below.
There is a link to the fix on this page: