[Plugin: List Attachments Shortcode] Trouble with Groups

Hi
Nice plug-in!
I am however having an issue with grouping attachments. When I add the groupby=”post_content” into the page, I only get one attachment per group, even though there are multiple attachments that have the same description?
Any ideas?

Thanks
Michael

http://ww.wp.xz.cn/extend/plugins/list-attachments-shortcode/