Is there more to using natural brushes vs synthetic brushes?

I’ve just gotten my first natural brushes because I was tired of dealing with synthetic “hooks. But I find it impossible to make these brand new natural brushes keep a tip as I paint with them. As I paint, they quickly split into multiple points and otherwise spread out, rendering them useless! I don’t let paint more than halfway up the bristles, I rinse it out every few minutes, and I wash with brush soap when I’m done. I have an Army Painter brush and a Windsor and Newton brush, and they both have this same problem. What am I doing wrong?