Thanks for reaching out, Helen!
If they are logged in on their accounts, and they do not see the option to "start" their assessments, then this indicates that the email addresses on the assessments do not match the ones they used to create their accounts.
Please, double check the email addresses on the assessments versus the ones they used to create their accounts.
You can then either change their emails on the assessments, or they can add the alternative emails to their account--by logging in, clicking on "Account Info" link in the top right corner and adding an email alias in the alias field.
This will then populate the surveys to their accounts.
Please, let me know if this helps or if there is anything else I can assist you with.
Tom with Wiley