So I just saw on 2.6.9 this bugfix
21) [Bugfix] WHMCS would not create VMs if the master was locked. This is fixed now.
I tried unlocking the master, however I still got the cannot create vps on locked server.
I experimented and on slave config on master I set the max number of vps's to 5 for eg hoping it would force it over to slave but now I get.
Order Accept Encountered Problems
Errors : VPS creation limit exceeded