I have several citations with 3-6 authors. In APA 5 and 6, the first occurrence should show all authors names and subsequent occurrences should show only the first author with “et al.” at the end. This is in fact how the output style is set up under Citations > Author Lists. I have been using EndNote X for years and it has always behaved correctly in the past. But in my document today, it is treating all citations as a subsquent citation, even if it is really the first citation. What might some possible reasons for this be? I am not aware of anything I am doing differently in this document, but apparently I must be.
Are you sure you are using the correct style for the document? Have you updated Endnote and are now using a “updated” perhaps incorrect version of the style?