Plugin stuck on Error downloading smushed file for attachment id: 113531

I'm using this plugin to smush my existing media.
4153 of 17667 attachments have been sent for smushing.
3956 of 4153 sent attachments have been fetched.

The plugin won't allow me to continue past displaying this message:

Error downloading smushed file for attachment id: 113531

Is there an option to skip this attachment and get on with the rest of them?

Thanks
Keith