Some references are correctly cited as et al., others are not

Hi, I’ve spent a long time trying to find an answer to this - both for this same problem on  this paper I’m writing and on the previous one. I’ve actually found a work-around whereby I just put in the wording I want as a prefix and use the ‘Omit Author’ option. 

I am writing a report, and using Endnote X5 (in Word 2010). I am using APA 6th and have altered the style so that et al. should appear every time there are 3 or more authors. Usually this works, but sometimes it does not - just at random times for random papers - ie: not just on the first apperance. 

Is there an easy fix? I am finding this whole thing so cumbersome and time consuming. Much more complicated than the last time when I used Mendeley. 

Screen shot attached of adapted style. 

Thanks.