Once you have assigned a particular test by using your credits = the same test. You can reassign it to be completed again but it will be exactly the same.
All questions answered incorrectly by a student are returned to the practice question pool and can be attempted again at a later date.
To retry questions, simply go to the ‘Dashboard‘ in the ‘My 11plus‘ area of the website and press the ‘Retry Questions’ button.
I hope that explains things to you. Have a look at: https://www.11plus.co.uk/coast-help-tests/