My dog loves playing ball. But... she plays keep-away or drops it and snatches it right back up.
Our lives as humans are loaded with conflict between what we want to do, and what we've learned to do. Wasn't it just last week that you thought: If I eat all that delicious fried food, experience says that I'll feel sick in a few hours. And I'm not even hungry, and it's expensive. But it smells so good! Just one more bite... It was completely natural for our ancestors to love high-calorie foods - we have inherited that love, even though it doesn't serve us well anymore.
Your dog is feeling the same kind of conflict. I know from experience that if I drop the ball, he'll throw it again, and I love that! But it's just perfect to chew on... When we play fetch with our dogs, they love it because the ball acts like a fleeing bunny. It was completely natural for dogs' ancestors to love chasing and catching bunnies - dogs have inherited this as a love of toys, even though it doesn't always serve them well anymore.
How can I ask her to drop it and override the system?
The easiest way is to bring some delicious treats when you play fetch. Say "Drop It Please!" with the treats hidden behind your back. Count to two in your head, and then show her the treat - put it on her nose if you have to! After she drops the ball and you pick it up, she gets the treat. Keep at this until she learns the game, and starts dropping the ball during the time you are counting silently in your head. When she's had a few days of practice, you can fade out the food treats if you wish to, by sometimes offering them and sometimes not.
Or, you can just grab a bag of your favourite guilty snack, and enjoy watching her keep-away game. It's a peek into her amazing and distant past, bubbling up right in front of your eyes.