What is the endnote output style you are using? It seems that there is a difference in the way - first and subsequent appearing citation settings are set in it. The subsequent box is missing the space. See where the setting is in the image attached. Edit> Output Styles, edit “your chosen style” then go to Author Lists and insert the space before " et al" in the susequent Appearances box. – then SAVE AS to a new name (if you can’t just “Save” it, do not use the same name, or there will be two on your computer and endnote may random use the old rather than the new version). If you were able to just “save” then go ahead and do so and the change will be reflected in your document if you “update citations and bibliography”. If you needed to save as to a new name, then you will need to change the output style used in the document (in the word processor, not in the endnote program). you will probably need to choose “select another style” from the dropdown, since the newly edited style will not yet appear in the dropdown list.