If a question is being marked too generously or too harshly, the fix is almost always the mark scheme rather than the individual marks.
Read the marking first
Open a marked answer and look at the Marking breakdown. Each criterion shows whether it was awarded and quotes the phrase that earned it. If the marker credited something you would not have, the criterion is too loose. If it credited nothing where you would have given a mark, it is too tight or it is asking for something the question never required.
Then fix the scheme
Open the lesson from My Library and click into the question you want to work on.
Edit the Mark Scheme and save.
Then re-mark the work that was already submitted, so the whole class is judged the same way. See Remark free response questions.

