Terviiix Pencil Flat Iron, Small Flat Irons for Short Hair, Beard and Pixie Cut, 3/10 Inch Ceramic Tourmaline Mini Hair Straightener Dual Voltage with Adjustable Temperature, Auto Shut Off

It's caused by being brushed too roughly. The strands stretch out and become kind of crinkly. Boil washing or flat ironing it can usually help. Use caution since too much heat can melt the hair. Always do a strand test first. If you go with the flat iron approach, I'd recommend this one. (Or something with the same features. I'm not affiliated with the brand, it's just the one I use.) I think it's the same one recommended by Dolls2Remember on YouTube? It's smaller than standard flat irons, so it's great for working on doll heads, and it has a digital display with adjustable temperature settings. I usually start at like 180F and gradually increase the temperature until I start to see results.