Did you check the “Ambiguous Citations” section of your output style (go to the toolbar, select EDIT, OUTPUT STYLE, EDIT [name of style])?
The Ambiguous Citations has a setting for adding a letter after a year (see image). This option should be checked. You could also select the first option to include author’s initials or full name to distinguish authors with the same name. Close the style’s window when completed. If prompted to save the style note as a Copy, remember to change the output style in both EndNote an your wordprocessor or the changes won’t take effect.
