[photoshop-beginners] drop shadow

I've made a background for a web page. It's just a plain image with a faux left column that has a drop shadow. But the drop shadow doesn't seem to go all the way to the top and bottom edges of the drawing and so when it repeats, it has lines in it.
Here's the page
http://www.nmlocal.org/
How can I make it repeat smoothly?

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