I believe you have to be on an AA-marketed and AA-operated flight, based on the fine print on AA.com: "Upgrades on American are valid on American marketed and operated flights". See
https://www.aa.com/i18n/aadvantage-p...e-upgrades.jsp.
If this was a CX ticket booked under CX flight numbers for the AA legs, the AA legs were CX-marketed and thus not eligible for upgrades, unless I'm misreading the fine print. If this was a CX ticket yet the AA flights were booked under AA flight numbers (CX can sell tickets on AA flights per the interline agreement, even if there isn't a codeshare), the AA legs were AA-marketed and thus eligible for upgrades. How exactly was the ticket issued?
And, AA.com is sometimes wrong. I had a flight a few weeks ago show as being "upgraded" when it fact it was not. Somehow the web site showed my seat on an already completed leg for an upcoming leg. It was strange and clearly a bug. The programming leaves much to be desired.