[Upfront] Submenu is transparent even if it shouldn't be

Hi,
The submenu if my main top horizontal menu works fine on some pages, but not OK on a few of particular pages. It looks like it would be transparent when:
- I open a page made with Single Page Teden template.
- I didn't scrolled down yet. If I had the problem doesn't appear.
What is wrong? How can I eliminate this?
Thanks.