Hi Wally,
Are these NFS or VMFS datastores? Are you logging into the service console with the root account? The most likely case is that something has a lock on the vmdk file (a vm that is powered on references the vmdk file). If you are using NFS and happen to have taken a snapshot, you can always scan the vmdk files in the .snapshot directory. You should also be able to take a VM level snapshot, perform the scan, then release the VM snapshot.
Hope this helps,
-Eric