Not automatic charging

Hi,
When my robot is finished cleaning. He goes to the dock and clean himselfs. Till there it's fine. But then he stays 1cm out of the dock and doesn't charge.
When I click on the app on the home button. He goes out of the dock and in again and then he is chart.
Anyone an idea what is causing this.
It's from the beginning that I bought it new, he is not charging on his own.