It is most likely that you have the authors in the various records differently. If you copy that name from one, to replace all the others, endnote won’t think they are different authors and “diambiguate” them. R D Stacey, Stacey, R D, Ralph D Stacey or Ralph D. Stacey (with the period) are all interpreted as unique authors. they all need to be exactly the same. Sometimes even a double space or an extra space at the end can trigger this.