I wonder, though, whether the author is entered differently in each reference – in which case EndNote will treat them as different authors, and so not add the letters to the date.
Check to see that the author names are consistently entered into your EndNote library, including spacing and punctuation. The easiest way I’ve found to do this is to copy-and-paste the authors from one record into the other.
More about author consistency at http://endnote.com/kb/82224
Cheers
John