A Montessori method teacher's role is to guide a student to choosing appropriately challenging work, that both helps the student develop their skills and expands their horizons appropriately, while still helping cultivate an interest in learning within the child. Child led learning means that the child chooses what they work on to some extent, but the teacher is guiding them to what is an acceptable choice.
From my experience as a Montessori parent, this is not an uncommon issue, and one a teacher should be well able to handle. My children are younger than yours, but both went through phases where they picked the same thing regularly because it was comfortable. The way their teachers approach this is to allow that sometimes, because being comfortable is not a bad thing per se; but to limit that to only occasionally, and instead suggest them pick more challenging things. One approach that's effective there is to encourage the student to teach the comfortable activity - if they're comfortable with it then of course they're likely to be good at teaching it, right?
One thing I'd also note is that not all Montessori approaches are identical - there are at least two different major Montessori bodies that I'm aware of, and they don't do exactly the same things.
A few references that may be helpful:
From NAMC, a Montessori teacher training site:
Montessori’s phrase “follow the child” does not mean you let the child do whatever she wants. Rather, it is an acknowledgment that the child has her own pattern. The key to understanding this idea is observation. As Montessori guides, we constantly observe the child in the classroom. We pay careful attention to her interests and the activities to which she is naturally drawn, and we monitor her understanding and development of skills. We then use this level of ability as our guide.
The point here is that the Teacher (called a "guide" in a lot of the literature) is responsible for the child's path; it's not just the child choosing on their own. It's a matter of observing the child and seeing what they can do. Letting them choose is part of the program certainly, but the choice is not unbounded.
From AMS, one of the major Montessori bodies:
In this way, the teacher serves as a guide rather than a giver of information. She prepares the classroom environment in order to support and inspire the developmental progress of each student and guide each child’s learning through purposeful activity.
In AMS, the aim is for the teacher to organize the environment appropriately for the children in the class, to help them find activities that are appropriate. AMS is a bit more hands off I think than some of the other Montessori concepts, but it's certainly still expecting the guide/directress (another word for teacher here) to do some "guiding" in addition to the child directed learning.
From AMI, another major Montessori accrediting body, a teacher:
know[s] when to intervene, and above all, they know when to step back.
Respect children as individuals with unique plans for development.
Assist children to progress at their own pace and gradually discover their potential while helping them to help themselves.
The focus again is on 'unique' plans for each child, and teaching them how to learn, so they can choose good lessons for themselves most of the time.
I will say that my experience here is that the teacher makes a huge difference. We're lucky to have a great teacher, who understands our (five year old) son very well and helps him make good choices - but it definitely works. I've seen him work, and he talks all the time about choosing challenging work and is always excited to get new lessons. But with a less good teacher, I could imagine that wouldn't go as well.
My suggestion is to talk to the teacher, and observe in class some if you can. You may not be getting the full information if you're just relying on your daughter's feedback; the teacher may have a very different understanding of what's going on. But perhaps it's possible your daughter simply needs the same lesson repeatedly - it might be hard for her! Especially some of the math lessons can be repeated for quite a while, in my experience, for students who truly do have difficulty with a concept; not three months usually, but for weeks at least.
