How to solve the problem of parallax with cross-browser compatibility?

Question

Cross browser

Hello!
There is such a page on which there is (should be) a parallax picture.
Parallax works in Safari 9.0 and Firefox 38.0.5, but it doesn't work in Chrome 44.0 and Opera 30.0. I'm even afraid to open the site in IE 8,9,10.
Parallax code in CSS:

.rus {
    background: url(img/redsquare.jpg) no-repeat center center;
    z-index: 2;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
    background-attachment: fixed;
    padding: 30px 0;
}

In PHP parallax is wrapped in a section:

<section class="rus ruover">
    <div class="section-title text-light"><h2>Nabízíme níže uvedené typy jazykových kurzů:
        <!-- <span>but <em>design</em> 'aint one</span> --></h2>
    </div>
    <!-- <a href="#portfolio" class="down-arrow-btn"><i class="fa fa-chevron-down"></i></a> -->
</section>

I have not been able to solve this problem for a long time. Anyway, I'm stuck somewhere.
Guys, tell me what could be the root of the problem?
Thank you!