Error 1 return on command ATND

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Hi I am using Xbees serie2 connected in API and it works pretty well.
There is an issue I cannot solve. I receive most of the time 1 as a feedback to ATND command to the coordinator. I did reset the network several time. The coordinator keeps failing and some radios cannot join.
I cannot find any documentation to troubleshoot that.
Any thought is welcome!

How many of your nodes are set as End devices? If you are exceeding your total number of children allowed per parent, that will keep them from Joining.

I have one coordinator, 4 routeurs and 2 end devices in my network currently.
I also forgot to mention that the information is still flowing.