She wasn't broody long enough. It can take a couple of weeks for the proper hormones to actually get rolling. Sometimes more. I don't usually try to place chicks until a due date or after.
If she's a first time momma she also would have beginners trouble with chicks.
I never put fewer than six under a first time broody turkey and never less than four under a first time chicken hen.
They make mistakes, they fail to recognize them as something needing care, they step on them, they squish them. It happens with first time broodies.
Give her more time and offer her babies again. Try for younger ones, week olds can be restless and hungry and scoot out from under before she's accepted them completely. Day olds huddle and are more still and stay hidden, helping with acceptance. Week olds are hungry and thirsty and restless.