making change in site.css does not make any effect why

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MVC

In mvc 3 razor view I'm trying to make changes in the my header table. the following is the one in the site.css.

table th {
    padding: 6px 5px;
    text-align: left;
    background-color: #e8eef4;
    border: solid 1px #e8eef4;
    font-weight: bold;

}

I tried changing colors but it does not do anything but it stays the same. No changes are made why is that...
but making changes in table element ( in site.css) get affected.

table {
    border: solid 1px #e8eef4;
    border-collapse: collapse;
    font-size: 120%; width: 100%;
    border-spacing: 0px;
    background-color: #e7e7e7;
    color: #666;
    clear: both;
    

}