The flight is with an AA codeshare number (at LHR I transfer to continue my trip to the USA with AA).
So, I called, twice (Turkish and UK numbers) and they tell me the reason is that this is a AA codeshare, otherwise the ticket is fine. I know that's not true, because I have done this many times. Any ideas what may be going on?
Is it a ticket issued by BA or by AA? If AA, call them to ask if anything is wrong with the ticket.
You may also need to use the record locator for the prime code (which is not normally revealed to you) rather than the codeshare record locator that you have now. BA will tell you it if you ask (and they're clued up enough). However, OLCI to a codeshare should work...