Does the Hopper not scan for Delta? I loaded it up and it did not show one Delta flight for me.
Almost no apps scan American, and Delta and United are frequently left out as well, as they use very extensive integrated booking systems, linked to international carriers (OneWorld, SkyTeam, and Star Alliance) that cost real money (in the case of American, IBM money) to connect to. You'll have to visit their websites periodically.
One thing I do is go to the airport's Wikipedia page and check a list of destinations and on which airlines. Once I have the very few airlines picked out, I go to their sites individually. Since American, Delta, United, Alaska, and Hawaiian use fare buckets, their prices almost never go down and will always go up as you get closer to the date of travel. Southwest, Spirit, Frontier, and other LCCs use a different pricing structure involving fare sales from predicted and actual sales and load figures, and JetBlue is a hybrid system, so those are the ones to track if you care.
And if you fly a lot, you should check which airlines service your normal routes the best and go full bore into that one airline. The perks you get once you hit 25,000 miles or so with a single airline in a single year are substantial and worth real money, and have saved me thousands over if I had always gone for the cheapest flight. 25,000 is hard to hit with domestic flights, but even a single long international sector can put you over easily, and almost all of the overseas carriers will credit your domestic mileage program. For instance, when I flew Cathay JFK-HKG (yeah, that's a loooonnnnnnngggg flight) I booked on Cathay but got 20k+ mileage credit on American. Flying SAA JFK-JNB is another great example, you can get United miles from it and that's a 16k mile RT nonstop.