Different children, different people, worth trying gently to have an open conversation or 2 or 3 to provide a space for her to say if there is anything, in particular, bothering her, but it may just be the way she is, I know my first at age 5 was not ready for school and the separation, but by statutory school age he was. Second child did none of them trying to separate from her, that I had done with my first, waited for util she was ready and would you believe that she was more than ready at 5, in fact, she'd have been ok earlier, but there is such a lot they can get out of being one child with mum and doing all the fun things you can't do when at school.
Both went on school trips, holidays with friends, then to university, then travelled the world.
I often found that if there had been a problem at school, the summer break seemed to help them grow up and chill out and by school return time the problem had disappeared.