Can't move some VMs into a folder

In vCenter 5.1 (either the Web Console or the VI client) I can move most VMs in the 'VMs and Templates' view from the 'Discovered virtual machine' folder to another folder that I have created.  However, some other VM's will not move.  I get the red circle with the black X when I try to drag them to the folder I want.  I can't see any difference between the VMs.  They are all Windows 2003 or 2008 VM's with VMware Tools installed, although in some cases it is out of date.

What am I missing?

Thanks,

dan