I need assitance with this adjecent sibling selector

Question

If this were my HTML body and CSS (example)

nav ul li a {
  color: #333; 
}

nav ul li a:hover {
  background-color: #ccc; 
}

nav ul li a:active + a,
nav ul li a:focus + a,
nav ul li a:hover + a {
  background-color: blue; /* Highlight the background color of the link immediately after the active link */
}

<nav>
  <ul>
    <li><a href="#">Home</a></li>
    <li><a href="#">About Us</a></li>
    <li><a href="#">Products</a></li>
    <li><a href="#">Contact Us</a></li>
  </ul>
</nav>

The part where comment is, it's not doing excatly that, why?

I mean I thought that there might be a problem with some rules overiding but that's seems impossible, I just don't seem to find the problem.