Intriguing, Jazzo...I perused a half-dozen websites on the matter, and it is, indeed, an ongoing mystery due to a lack of solid evidence in any direction. Here's the most comprehensive discussion on the discrepancy I could find. It seems that "Success" came to be generally attributed to Emerson, but there's a similar Robert Louis Stevenson piece, and then there's a an unknown poet, Bessie Anderson Stanley, whose great-grandaughter claims it was hers. Stanley has received more attribution for the work than Stevenson, it seems. And more extant evidence supports the theory of her authorship.

http://darriendesign.com/serendipity/whodunit.htm