How to debug Sonoff ZBMINI which won't pair?

i’ve bought several Sonoff zbmini swichtes and some of them paired using the HA Zigbee integration without any issues, just powered them up, add device an where found nearly immediately. But some device won’t, and i’ve tried (feeled) a hundred times, the green LED only flashs regulary.

Two questions here:

1.) When does pairing occur?
I can activate pairing by push and hold the actor-button on the zbmini for more than 5 seconds. Then the green flashing light will go off for a short and start blinoing again. Then i release the buttpn and push “add device” on the Zigbee integration of HA. I also tried the other way, by fisrt start adding proc and the set zbmini to pairing.
Both without any noticable difference.

2.) How to debug pairing?
I once used a RF sniffer to find out how often and how long the zbmini tries to send put a beacon and it looka like it does this only for some seconds right after entering the pairing mode. But maybe this is a two players game and the sender needs some activation from the coordinator?
I also can’t make any use put of the logs written during pairing. How can i find out which address my non-pairing device has, to find out if it might be seen but not paired from some reason?