proxmox permissions pool,user,group - how to verify missing ?

[immo]

The pool permissions are an easy way to assign permissions to a list of VMS to a list of Users (Group members)

But a member A of group B can not select an iso image to boot as CD in a VM which is part of pool C.

So how can I determine which rights are kicking out which ones? The Group B has PVEVMAdmin rights at the Pool.