I read about it in the codex: http://codex.ww.wp.xz.cn/The_Loop#Multiple_Loops_Example_3_-_Plugins
“It has been found that some plugins don’t play nice with multiple loops. In these cases it was discovered that some plugins which deal with the keyword(s) and tagging of posts, only work for the first instance of a loop in a page where that loop consists of a subset of total posts.” And of course it goes on.
moshu, thanks. I can’t believe I was so blind!