Some devices don't get response after Join-Request

Recently we have been experiencing that some of our newly added devices never get a response after their Join-Requests. Like nothing at all, not even a MIC-error. Nothing is shown in the device view, not under frames nor events. I tried deleting the device, and recreating it, but still nothing.

I tried deleting an older, existing device, and then re-creating it in the system, and that device got a Join-Accept as expected after the Join-Request.

We are running v4.

do you recieve any uplinks? from nodes? then devices are not joined at all.what activation are you guys using?