Clear chrome cache or open test in incognito mode.
Check maximum allowed test attempt. If test attempt limit is set to one and if student has started the test anytime before, he will not be able to take the test again. There are two options.
You can clear the student test attempt through your admin dashboard. Search for the student - http://support.learnyst.com/manage-your-learners/search-and-filter-learners/search-for-individual-learners-and-see-courses-enrolledbought , Then click on student record, click on test tab, click on the particular test and click on 'Reset Student Test' button.