It’s difficult to know exactly what is going on without being there but I can only speak from experience in saying that if you’ve been with her long enough to have transitioned from red balls/orange balls to regular balls, then it’s most likely partiallya factor that she has been learning in a group, or semi-private environment. It can be difficult to fully get the point across when she’s having different issues than the other girl with her in the lesson.
My advice would be to figure out what type of learner she is and really go from there. If verbal instruction hasn’t worked with her, start demonstrating and physically doing things for her a lot more often than you have been. If you already have been doing those, try the one you’ve been doing the least, unless you already know what type of learner she is and have honed in on the area.
As you stated indirectly, she is improving when you give her drop feeds and that’s part of the process. At 8 years old, and at any age for that matter, things will be a little different when comparing a drop feed, to a toss feed, to a racket feed and finally actually hitting. They will all progress at different rates.