All you have to do is "ring" the mouth one time with a goodie, and the dog will begin really TRYING to catch.
Get your dog panting, get some good treats, and toss for the open mouth. Eventually you will be successful.
I have found it helpful to show the dog the treat, and make a slow motion tossing motion once or twice so the dog focuses on your hand and the treat, and then toss on the 3rd time. Toss with an underhand motion so the food makes a little arc ending at (or IN if you are lucky) the dog's mouth.
Keep the dog on a leash, and DO NOT let it pick up pieces off the floor that it misses.
This one exercise will tell you how important time rewards is in training dogs, because most dogs "get it" after they catch the food the very first time.