Let's see. The center tag has been deprecated. I know, it really doesn't make a lot of sense to me either, even though I understand the subtleties of why it wasn't really working out. To make matters 'worse', the way to center things now is to set their margin to
margin: 0 auto; in the stylesheet.
And that's just the short answer. But I would try getting rid of the center tag and putting this in the head of your page:
And if you don't already have a valid URL DOCTYPE as the very first thing in your page's source code, you will need that too. If that's a mystery to you, let me know, I think I can explain that as well.
margin: 0 auto;
But things would be easier in any case, if you want more help, if I had a link to your page, if so:
Please post a link to the page on your site that contains the problematic code so we can check it out.