Perhaps start by verifying that they are task Collaborators (the @mention should ensure that, but just want to be positive).
Then determine if the notifications went to their Asana Inboxes; it should have.
Any chance the email notification went to spam?
Those are a few things to try. You may then want to ry these troubleshooting steps, then please create a support ticket: temporarily see Try the new Asana Help Center! - #55.
Also, may I move this topic out of this private category so 700k+ others can benefit from it?
Thanks,
Larry