Hi Bidyut,
What is happening is that while you are changing the states of the checkboxes back to normal when the first incorrect answer is made, Storyline still sees the original answers as selected. And because the Submit button does not get enabled until at least one checkbox is set to selected, the user by clicking on the original answers actually is deselecting them. In any case, they are still selected, even while the checkbox does not show that. (That means if they chose items 1 and 2, and then went back and clicked 2 and 3, Storyline would see 1, 2, and 3 as selected.)
I'm not aware of a way to clear answers without restarting the screen (which would undo the count of attempts), but maybe someone else knows of a way.
The good news is that if you remove those triggers that clear the checkboxes, the learner will see what was checked originally, and can make changes and resubmit.
