Did you enter the authors on separate lines?
If you have multiple authors in your reference in the EndNote library, make sure you have placed each author name on its own line. If you have not done so, be sure to enter a carriage return after each name. There should be no punctuation between each author as the Output Style will include this information:
Argus, Michele V.
Matthews, Edward K.
If this does not resolve the issue then I would suggest contacting Technical Support.
1-800-336-4474 option 4 then 1
or