Being a native speaker is no guarantee of a top grade - I say this as an MFL teacher of over 30 years.
Some of my Y13 class last year did get A and A* grades - not because they are native speakers, or even because of anything that I did, I suspect - but because they worked their socks off.
I had 2 groups for different languages, so no bias towards one particular language, and I work in a state comprehensive.
They did a lot of work in their own time , which extended their vocabulary. They had an amazing grasp of grammar, and they were utterly fearless in the oral exams. They did an extra literature essay every week, of their own accord.
In contrast, the last native speaker I taught,some years ago, got a B grade. Sloppy grammar, and lack of awareness of what is needed to get a good grade - he was adamant that he didn't need any support from me at all.
It's also worth having a detailed look at the examiner reports for the exams, as they do give tips on what the best students do.