Yes, that is it. I recently tinkered with the center thing trying to figure things out...doing with quotes, without, taking it out entirely, etc....ive exhausted all the options I'm aware of it appears
Here is the code used in the header for buysell ads
<!-- BuySellAds.com Ad Code -->
<script type="text/javascript">
(function(){
var bsa = document.createElement('script');
bsa.type = 'text/javascript';
bsa.async = true;
bsa.src = '//s3.buysellads.com/ac/bsa.js';
(document.getElementsByTagName('head')[0]||document.getElementsByTagName('body')[0]).appendChild(bsa);
})();
</script>
<!-- END BuySellAds.com Ad Code -->
Here is the code in my sidebar for the one not working
<center>
<div id="bsap_1246770" class="bsarocks bsap_2a20be308405721add23245432fe79f8"></div>
</center>
Than I have my header ad code which is similar to the code you posted and that seems to be working fine.
<center>
<div id="bsap_1246687" class="bsarocks bsap_2a20be308405721add23245432fe79f8"></div>
</center>