my image did not placed as a full background img in html page

this the code that i have written and it didn't work , my problem here is that code work but the image did not appear as it supposed to

 .about-bg{  
   background: url(../img/about.jpg) no-repeat top center fixed !important;
   -webkit-background-size: cover;
   -moz-background-size: cover;
   -o-background-size: cover;
    background-size: cover;
    height: 100vh;
    width: 100%;
   background-position: top center;

   }

this isn't go well