First off, AA does not fly its own metal to TPE. It is actually operated by EVA Airways.

My guess is that you have an AA ticket stock, with an EVA codeshare flight.

If that's the case, perhaps you can contact EVA Airways directly and tell them your first and last name, the day and the city pair you're flying on the EVA operated flight (without mentioning your AA confirmation number or the AA codeshare flight number) and have them look you up in their system.
Then you can ask them if they can attach your OnePass number to your record in the system. See if that works...
Some operated airlines can be very particular of whom you purchased your ticket from, in determining whether they want to award miles.
Case in point... forget about asking MH to award you WorldPerks miles for an Air India ticket stock but operated by MH.