So you haven’t said what area she works in or her grade. Saying she’s in management tells us nothing as that could still mean many things. For example I am a G7 in digital and manage 3 people. An HEO in ops might manage many more.
If you could provide some details you might get more specific advice. Is she trying to move up within the same area, for example becoming a team leader in operations? Is she trying to switch profession, for example moving from operations to policy? What grade is she now and what grades is she applying for? This information would help people give better, more tailored advice.
In the absence of that information, here is some general advice.
Firstly, the civil service is generally supportive of helping people progress and she should be discussing her ambitions with her manager. There is a lot of support potentially available, for example arranging mock interviews.
She should have received scores for all her applications and interviews, and should be asking interview chairs for feedback. Has she been doing that? The feedback will tell her how far off the mark she is. Again, it’s hard to give specific advice without this context. For example we don’t know if she’s just missing the mark, or falling far short, or scoring enough to meet the standard but being beaten by other stronger candidates.
Lastly the absolute best thing any civil servant can do to improve their interview technique is to be on a panel themselves - you learn so much being on the other side. I have arranged this for people I know need to improve their interview techniques. In my department you must have one panel from outside your business area so I look out for people in other business areas who are hiring and ask if my staff can do this.
Also a pp mentioned that jobs have to be advertised externally. I’ll just clarify that only applies to permanent roles not EOIs.
OP, I have known some civil servants who do not interview well, decide the system is against them and choose to complain to others about how unfair it is - rather than appreciating and using the feedback they receive, and engaging with the many, many sources of support available to improve interview technique and experience (line manager, e-learning, holiday cover opportunities, etc). I don’t know if your daughter is one of these, but don’t let her become one.