The page only lists requests that are still Open and whose Role Needed is either Any Role or one of the roles on that employee's Roles list. Check that: the request has not been filled or deleted; the employee page has the role added under Roles; and the employee is logged in with the user account linked to the employee record. If they click I'm Available on a request that was filled in the meantime they see "This shift is no longer available.", and a second click shows "You have already signed up for this shift."