ambiguous citations

Hi! I am having problems with ambiguous citations in endnote X4. I have 3 references with the same authors and year. The program add a letter to all of them, but this is done according to the alphabetical order of the references (Authors, year, title) and not according to where they appear in the text.

These are the references:

Facelli, J.M. & Pickett, S.T.A. (1991a) Indirect effects of litter on woody seedlings subject to herb competition. Oikos, 62, 129-138.

Facelli, J.M. & Pickett, S.T.A. (1991b) Plant litter: its dynamics and effects on plant community structure. Botanical Review, 57, 1-32.

Facelli, J.M. & Pickett, S.T.A. (1991c) Plant litter: light interception and effects on an old-field plant community. Ecology, 72, 1024-1031.

And the order of apperance in the text is 1991b, then 1991c, and then 1991a. I tried modifying the ambiguous citation part of the style I am using (which by the way is J Ecology (UK)). I select sort order Author + Year in citations and bibliography parts, but it didn’t work.

I know some other people has the same problem, but I couldn’t found the answer in the forum. Any ideas on how to fix it?

Thank you! Alejandro