Work Flow Rule by Updated Field Not Triggering.

Work Flow Rule by Updated Field Not Triggering.

We have a work flow rule that triggers when we change the owner of a lead. It sends an email to our customer services to let them know who has been given the lead.
 
If we edit each lead individually then it works but if we mass change owner from a lead veiw then then rule isn't triggered..
 
Why is this and am I missing something here?
 
Chris