See the following forum post as well:

http://moodle.org/mod/forum/discuss.php?d=51851

It seems like, when the Each attempts builts on the last feature is enabled, some questions are not getting the proper grade when a second attempt is made. This occurs even when adaptive mode & penalties are disabled. The symptons include the total grade being less than 100% even though all questions on the second attempt were answered correctly.

I discovered this problem when I tried to manually override a question, and found that its assigned grade was 0, although it was answered correctly. When I assigned the grade to 1 for the question, the grade for the entire quiz went up appropriately.

So, I suspect that grades are not being recorded properly for future attempts on quizzes when the Each attempts builds on the last feature is enabled.