I'm not entirely convinced this is just an Amex copr card and EX problem. There have been a number of posts by folks complaining that their other Amex cards like Gold and Blue are not reporting on EX. Contributor KDM and I are fighting with the very same issue. Amex blames EX and EX blames Amex.
I was also unhappy to learn that my AMEX Corporate Card does not show up on my credit reports. I have had the card since 2001 and have maintained an excellent payment history. I could use a boose to my FICO scores.