Child theme does not work

To not change the original style.css I created a child theme. But adaptaiones in the childs style.css are not applied.
What is going wrong with the child theme?
For testing I changed one small point in the original style.css but also these changes were not applied. This confuses me much more.
Only when I go to the customiser of the theme (additional css), the changes are applied. But I wanted to use my own child.
Any help?