When you are ticketed for one class of service, you cannot see seats in other classes. In the case of an intl. trip with PE sold as a separate cabin I believe it won't show up. Because the OP is only seeing selections in main cabin, it def sounds like an IT glitch that should be resolved with a phone call or twitter DM.
I believe American still treats Premium Economy as an Economy class ticket as you can't "upgrade" to Premium Economy with miles (and aren't forced to by PE to upgrade to Business with miles, etc.) since there isn't a "class of service" below that.