If she really is a good coach, try staying with her when you take lessons with an amateur partner. Maybe the one-on-one serious lessons were too much for her, but if she's coaching a couple, she'd be fine.
If that still doesn't work - like if by "flaky" you mean she doesn't show up for lessons or constantly cancels - or if you don't feel you're progressing how you'd like to, just explain that and say that at this point in your dancing, you'd like to try working with another coach. You can always take with another coach concurrently too, rather than totally break off with one and start with another.
Although dancing can turn personal - you're slathered against someone's body for hours on end, for heaven's sake - you are a customer, and you can take your business anywhere, whenever you what, for whatever reason you want. As long as you're not mean about it, the vendor (teacher) shouldn't get angry. They may be sad to see you go, or unhappy that they're losing income, or feel bad that they aren't able to give you what you want - but that's unavoidable. We teachers are human too

. But that shouldn't stop you from doing what you have to do for your dancing. Be respectful and honest wherever possible, and it should go fine. If an ex-teacher get snitty about it, well, they're not being very professional, and it's probably for the best that they're not your coach, since a coach's job is to support you, not to make themselves feel good.
Good luck!