If you have an older American Girl doll that you’ve played with a lot, this is almost definitely something that has afflicted your doll: after brushing her hair a few times, the ends of her hair become a frizzy, tangled mess that no amount of brushing can ever seem to fix. Even when the doll’s hair is braided or hid under a hat, the volume of the frizz is a huge distraction, and makes your doll seem cheapened and ruined.
The official response from American Girl themselves is always something along the lines of, “just spritz it with water and brush it some more until it’s smooth”. This doesn’t work, of course. Their other solution is to send your doll into the doll hospital for a new head. That not only costs more than $50 to do, but your doll will return with an entirely new face, one that you might not like quite as much. And it certainly does not prevent the frizzy hair from returning.
Currently, and probably forever, the doll hospital does not offer the option of putting a new wig on dolls. They claim it’s because the strong glue on the wigs prevents them from doing so. This isn’t entirely true, because it’s possible to do that yourself; but that’s only an option if you have another wig ready to go. (You can learn how to re-wig your doll with my tutorial [here] if you’re wanting to go that route instead.)
But if all you want is to restore your doll’s hair to its (almost) original shine and smoothness, then you have several very affordable options that I can recommend and will show you how to do here.