why is she struggling, do you know? If she is in the top set, she is capable of it. She may not like the teacher or it may be that she has fallen behind with learning endings. Unfortunately, these have to be learnt off by heart and practised when you encounter them because you very soon encounter the next forms. Initially you don't move very fast so you can get away with not bothering too much about this for a bit and then you get lost.
The speed with which you have to learn these forms varies from course to course. Some have you doing 3 declinations in all cases by chapter 3 of book 1; Cambridge has just the nominative and accusative cases till you reach chapter 9 when the dative is introduced. That is it for declinations then for book 1, which covers adjective endings for those cases and the imperfect tense in all persons I think. My dd used a different course.
You can cruise for a while without doing all the rote learning and practise but at some stage it catches up on you. It is unavoidable really. If she has not been learning very long, she can still cover that ground. Maybe you can help with that systematic coverage. What might also help generally is using an extra exercise book. Practice makes perfect, it really does, at least it stops you having to think and think which case could this be, to which noun does this adjective refer and so on. My dd used the following book alongside her course material. The exercise book followed a different order of acquisition but is simple to use on your own without a tutor or parental guidance IMO because it spends a long time on nominative/accusative till that has really sunk in and perhaps 3rd person sing and 3rd person pl of 1st and 2nd conjugation verbs rather than all forms of 1st conjugation verbs at once.
This book: Latin Practise Exercises
The book has no answers in the back. You can either purchase an answer book but I think it is really not necessary at this stage. Grammar is explained before it is practised, little chunks of grammar are practised over and over, very clear and straight-forward.