Twitter Bootstrap Grid Stacking Issue - html

Question

Evening all,
I'm having a little trouble with the grid system and am looking fir a bit of guidance here.
Take the following code for example:
<div class="container">
<div class="row"> <!-- PROBLEM -->
<div class="col-md-12">
<div class="problemclass">
</div>
</div>
</div> <!-- /PROBLEM -->
</div> <!-- /container -->
</header>
<main>
<div class="container">
<!-- WORKS -->
<div class="row">
<div class="col-md-4">
<div class="notproblem">
{CONTENT}
</div>
etc...
As you can see, I've marked a row as PROBLEM in the header and it contains a 12 column span. The CSS for problem class is set at about 300px height and a background image (It is set at 300px because the background image is 300 px in height). the code looks like this:
.problemclass{
height: 300px;
background: url('background.png');
}
Everything stacks and lays out perfectly... until I size the browser down to test the stacking. At this point, the row marked PROBLEM will begin to completely overlap the row marked WORKS; the content will overlap the background image, and the actual div goes under the background image! I have no freaking clue why LOL! My guess is that maybe I cannot have just a background with a set height and expect it to work?
Any advice would be appreciated, and I hope I was clear enough.
Also note that there are 2, 6 column grids above the 12 column grid in the same row that I did not include in attempt to keep the code a little more clean and clear.