That doesn't always work because the theme you're using may not have a style clarified for "alignleft" in your style sheet. I have that problem too so I manually add what I told you above and it works. You can either do what I do or manually add a style for the "alignleft" attribute in your stylesheet.
Many of the WP themes are not compatible with the WordPress default styles so that's why WP is ignoring the "alignleft" attribute.
Hang in there. I know things can get tricky sometimes.
