I have always had great success finding flights here at FlightAware, but recently I went looking for IB7017 (an Iberia flight from BOS to LAX) and wasn’t able to find it here.
Under “Private Flight Tracker” I searched for IB7017, IBE7017
Under “Airline Flight Tracker” I selected Iberia and inputed Flight # 7017
I searched for flights between BOS and LAX and didn’t find anything arriving around 10:45pm (the scheduled arrival time)
Then I went to another website, FlightStats.com, typed in IB7017, and there it was. Apparently operated by American Airlines, AAL223, which does show up on FlightAware simply by inputting “AAL223” under Private Flight Tracker.
To date, we have mostly focused our codeshare support on regional airlines that operate under the flight # of a major airline. We are in the process of developing a new feature that will detect a codeshare between two major airlines (like you described) and show the correct flight. You can expect that to be on the site in the next month.