I have 2 identical Cisco 2690x with flex stack plus modules installed in both. I have provisioned the first and it is up and running. When I add the second to the flex stack module and turn it on, it powers up and the flex stack indicator is green, but it doesn't seem to create the stack. In the CNA, the secondary switch is not even showing in the list.
You need to:
Remove the current configuration; in config mode for both switches, remove the provisioning config, on master:
no switch 2 provision ws-c2960x-48lps-l
Power off the switches. Remove and reseat the stack modules. Power up and reboot the switches with no stack cables connected. After reboot has finished, set the priority of the switch you want master:
switch 1 priority 15
Now connect stack cables port 1 master to port 2 slave and port 1 slave to port 2 master. Slave will now reboot and the stack is formed.