I'm guessing she's inexperienced at this. They don't all take to it like ducks to water - as indeed not all other animal (including human) mums are cut out for it.
my broodies typically stay with their chicks longer than 6 weeks; the best one last year (whose whole brood is still with us, laying and crowing as appropriate) started drifting away from them from 3 months, and it was a few weeks more before she roosted back with the grown ups.
My broodies hatch within the flock. Like others, I have found some are good at it, some not so, and that it's impossible to predict on the basis of breed or age whether a given pullet or hen will be one or the other; if they feel the urge, you just have to let them try to find out. And they do...