@jpaul As a test place a dumb (unmanaged) switch between one of the troubled target computers and the building switch and try it again.
If this works, then you need to get with your networking admins to check the network switches to ensure that (if) spanning tree is enabled, you are using one of the fast stp protocols (RSTP, MSTP, fast-STP, or what ever your switch port manufacturer calls it).
But, back on your original post. You can either define the VL Key in the proper field for the host, or you can define it in the global key configuration in the fog configuration->AD (I believe). You can also use the FOG group set function to assign a key to a group of existing computers. This will update the key in the individual host management page.
As you can see there are a number of ways to go about doing what you need.