Hello santhumax, I would update BNA to the latest version that is supported by your switches.  Did you gather a switch log prior to adding the new switches to the fabric?
Hi All, I have recently added 2 new Brocade 6520 Switches into the fabric, i can login them and when ran Fabricshow command it's showing all the switches in the Fabric, also i can see these in the BNA Switch Group on Both the Fabrics,. But the new Switches status bar showing as "Not Reachable", also the Fabric A/B status also showing as "Not Reachable" Is this something do to with miss any configuring step at ISL? or on the Switch itself? attached screen grab of Fabric B, for your reference, it's same case with Fabric A as well. [IMAGE]
BNA showing new Switch but status "Not Reachable"