DD never got more than a tiny part in in her school productions throughout her whole school career despite being heavily involved in her local musical theatre youth group. It put on really high quality productions in which she always had a major part and her singing voice was reckoned to be exceptional by the professional artists who ran the group as volunteers.
However, every year when she auditioned for the school show she got the back row of the chorus.
Now I'm aware that there will now be some rolling their eyes and dismissing me as one of those deluded mums with overinflated ideas of their offspring's talents. I get that, there's a lot of that around. However, she won a sixth form place to study Musical Theatre at BRIT School and is now on the foundation course at Mountview. So independent top tier professionals in the business recognised her talent and offered her places on massively competitive courses. But the local comprehensive doesn't consider her talented enough to even have one line. (Bitter, me? NOOOO!!
).
Having turned myself inside out over the years to understand this, I have concluded that schools aren't really concerned about choosing the most talented and best performers for these things as the aim of these productions isn't to produce the best standard of performance possible, it is used as a tool to offer encouragement engagement and confidence boosting to the less engaged and motivated pupils, to reward good behaviour and, yes, at times to pat the drama teacher's favourites on the head!
So OP, recognise this for what it is. It's not a reflection on your DD's talent, the school have other motivations.
I would recommend your DD to put her energies into finding specialist musical activities and performance groups outside of school. She will find her talents and hard work and level of achievement get their proper rewards there.