bootstrap coloms skip a line on col-xs-6

I have a bootstrap grid with four items. My problem is that the item in position 3 jumps to a new line and doesn't show in a grid layout.

This is a screen shot of what is produced with the following code

<!-- Position One -->
    <div class="col-xs-6 col-sm-6 col-md-3">
        <div class="stock-grid-item">
            <a href="item1.html">
                <img src="/images/image1.jpg" alt="" />
            </a>
        </div>

        <div class="stock-grid-details">
            <p class="name"><a href="item1.html">Item One Title</a></p>
            <p class="price">$10.00</p>
        </div>
    </div>

<!-- Position Two -->
    <div class="col-xs-6 col-sm-6 col-md-3">
        <div class="stock-grid-item">
            <a href="item2.html">
                <img src="/images/image2.jpg" alt="" />
            </a>
        </div>

        <div class="stock-grid-details">
            <p class="name"><a href="item2.html">Item Two Title</a></p>
            <p class="price">$10.00</p>
        </div>
    </div>

<!-- Position Three -->
    <div class="col-xs-6 col-sm-6 col-md-3">
        <div class="stock-grid-item">
            <a href="item3.html">
                <img src="/images/image3.jpg" alt="" />
            </a>
        </div>

        <div class="stock-grid-details">
            <p class="name"><a href="item3.html">Item Three Title</a></p>
            <p class="price">$10.00</p>
        </div>
    </div>

<!-- Position Four -->
    <div class="col-xs-6 col-sm-6 col-md-3">
        <div class="stock-grid-item">
            <a href="item4.html">
                <img src="/images/image4.jpg" alt="" />
            </a>
        </div>

        <div class="stock-grid-details">
            <p class="name"><a href="item4.html">Item Two Title</a></p>
            <p class="price">$10.00</p>
        </div>
    </div>