重复时缺少div区域

i have a problem with my stylesheet. i have repeated div by looping (php)

This is the looping

<div class=content>
    <?php $db=new db; $sql=$db->query("SELECT * FROM tbl_news WHERE category='1' ORDER BY 1 DESC LIMIT 0,10"); while($r=$sql->fetch(PDO::FETCH_ASSOC)){ echo '
    <div class=box-thread>
        <img src="images/galleries/preview_'.$r['img'].'">
        <br />
        <h1>'.$r['title'].'</h1>

        <p align=justify>'.strip_tags(htmlspecialchars_decode(substr($r['contents'],0,200))).'</p>
    </div>'; } ?>
</div>      

but the problem is showed in the picture, there is missing div there. I dont know why. any div i set to "float:left" so it's auto go to bottom if there is reach the page's width. This is my css

.content {
    width:100%;
    margin:auto
}
.box-thread {
    width:20%;
    background-color:#fff;
    margin:2%;
    float:left;
    min-width:240px
}
.box-thread img {
    width:100%;
} 
.box-thread p {
    padding:5px 20px
}
.box-thread h1 {
    padding:0 20px;
    font-size:18px
}

Thank you for any help