3 column issue – pushing main content below sidebar

Hi folks

I’m having an issue with a 3 column layout. I have two sidebars, left and right, and a middle section with the main content. For some reason, the first post is showing up ok in the middle, but subsequent posts are being pushed down and displayed after the sidebars.

Any tips on how to resolve this? I’m using the Arthemia theme,although I’ve heavily customised it.

Here’s an example: http://www.thevillagehearth.org/category/parenting

Cheers
Charlette