Having the same problem, AA Plat for years. Always book through corporate, my past 30 days of flights and future flights (booked through AMEX Travel) will not let me edit my seat selections to anything worth while.

Im stuck picking middle rows at the front of the plane or occasionally window or aisle seats in the back.

I called and talked to a rep (aa plat / gold desk) who was kind enough to remind me she'd been there 18 years, and stated that unless it was booked with American directly i wouldn't get my plat privileges. To which i replied, I've been flying your airline for over a decade and never heard of any such policy. I told her to please pass along my complaint and that i would consider other airlines if this stays true (im moving so its really not an idle threat)