WP125 issue

I am a newbie to WordPress and installed WP125. I installed 6 ads in 2 columns. All six ads are showing, but column one has one empty space and the extra ad is in the fourth row. So instead of 2×3 I have 2×4 with row 2 column one empty and row 4, column 1 having an ad. What did I do wrong and how can I move it up.

Thanks