Adding more end devices on a Zigbee network with a nRF52840 coordinator.

Question

Hello. We need to add more end devices than default (10) on our Zigbee coordinator and that was solved with 
 zb_set_max_children( ZB_DEFAULT_MAX_CHILDREN ); 
 where ZB_DEFAULT_MAX_CHILDREN is 32. So far so good and I now can add more than 10 children, but when I try problems start to show up. 
 I can add several children (I don't have 32 but I can add more than 20) but if I try to communicate with the children (one message per child at the same time) things start to fall apart if there's more than 12 children. With 13 children I will get a response from 12 children but the 13th is unreachable. This isn't connected to a specific 13th device but seems to be pretty random and the worst part is that I don't get any error, the messages are just dropped. So there seems to be a limit of the amount of buffers used during data transfers and the 13th package is just thrown away (as I said above, I don't even get an error message) and I guess I need to increase the amount of buffers available for transfers. Question is, how do I do that? 
 
 If I add even more children the behaviour of the coordinator gets increasingly erratic so there might be other limitations that also has to be fixed? 
 
 And is it possible to go above 32 children or is that a precompiled maximum? This is on zBoss version 3.11.1.5. Regards /Martin