Is the VM showing as locked?  You could try to manually unlocking it.  I did this in a test environment a while back with some success after a vm was stuck in a "locked" state.  Of course the gui and engine should handle most of this for you, manually mucking around the internal DB can cause some pretty serious issues if you are not careful...

sudo su postgres

psql -d engine -U postgres

SELECT vm_guid, vm_name FROM vm_static WHERE vm_name='VM_Name_Here';

This should return a string such as: "0ec20854-e1ca-4e49-be87-a6cd36d40c18"

Reset the lock:

update vm_dynamic SET status=0 where vm_guid='0ec20854-e1ca-4e49-be87-a6cd36d40c18';