Error fetching smushed images after processing

Smushing a total of 2063 images.

Did 4 manually - worked fine
Ran the next 1000 - worked fine
Sent the next 1000 to to be smushed, they sent fine, got the email. Then went to fetch them and get the following error after leaving it about 2/3 minutes.
"Error downloading smushed file for attachment id: 408"

Let me know if you need any more info to sort this one out