Did you try the same smart list as an actual Smart List?
Without a screenshot or a detailed description, I can't help.
Silly question, but are you sure that the link is correct? Nick is outlining a slightly different issue, so it would help to confirm that the link is correct. Also, is it possible that Marketo tracking was removed from that link? If it was, then you won't show any clicks on the link.
Have you included the correct link?
If link is correct and still it does not works try using
Clicks link in Email
Email is your email
Link Contains www.xyz.com
Hope it works this way.
As others have suggested, check that you have the right email and the right link in the smart list. Also check the schedule tab and, if applicable, ensure people can go thought more than once. If the campaign is reacting only to some people and not to others, check on filters.