Thanks people, that was the problem 
There's an issue with my script though - it sometimes doesn't correctly identify the creator of a snapshot - it gets it wrong and selects someone else - the wrong person.
@LucD: do you have any thoughts on this? I know you've dealt with the general problem of finding the creator of a snapshot before e.g. http://communities.vmware.com/message/1290894.
Thanks in advance!
Regards,