First, make sure your clients click on the button within your invite (which contains a token) and use the email address to which the invite is sent (which should be populated automatically by the browser) to log into Intuit Link for the first time for the year.  This will ensure their Intuit Link account is "connected" properly to yours.  Often times, problems occur when clients try to log into Intuit Link for a new year using their credentials without first clicking on the button.

If this doesn't work, logging in directly for a second time would sometimes do the trick.

Give that a try and let us know how it goes.