Hi Sandra,
There have already been countless posts in this Moodle quiz forum on the topic of multiple choice, multiple answers grading. The problem is that different people have different views about the ideal grading system, and the built-in grading system cannot satisfy all of those often contradictory views.
If you could tell us exactly what you want to achieve, then it might be possible to help you. Please provide real examples, and say explicitly what grade you wish to give when students select all or some of the correct answers and all or some of the wrong ones. And do look at all the other posts on this same subject, where you may find the answer to your query...
Joseph