Child theme only works 50% – Vantage theme

Im new to all this, and just created a child theme earlier today where everything worked, i managed to overrule some codes from the parent theme, Vantage.
But when i wanted to change something later with the excact same procedure it did not work… I wanted to change the hover color of the main menu which looks like this:

}
.main-navigation ul li:hover > a {
  background: #000;
  color: #ffffff;
}

but it doesn’t affect the website, the parent theme still decides the color…
However as mentioned the child theme works sometimes for an example it overrules when it comes to the masthead color
What can i do to make it work?
Thanks ??