If you answer a Matrix question incorrectly, AND you have "display incorrect answer" turned on, when you hit check answer, the page immediately displays the correct answer, unlike all other question types, which continue to display the incorrect answer and allow you to correct it.
I made a demo quiz for you to see the problem. If you answer the first question incorrectly, move to the next one and then go back, you'll see what I mean. Quiz: http://azs.somenewfrog.com/matrixproblem/
There are several matrix questions that all work improperly at that test page.