If a student has completely neglected to cover an AO, a Level 6 would not be appropriate. We often find the best students keep all or most of the AOs in play in any given sentence or paragraph. To neglect an assessment objective completely would be problematic because they simply wouldn’t be answering the question properly.
Having said that, if say AO3 is dominant – like in the AS paper 2 questions, it doesn’t necessarily mean the essay ought to be dominated by AO3, it is all about how well contextual information is integrated into the wider argument, rather than giving us a history essay. Think quality rather than quantity – even where the AO is dominant!