The idiom 'to drop the ball' means to fail to do something that you agreed to do or you were obliged to do. To 'drop the ball' can also mean to make a mistake or to be careless or fail to avail of an opportunity that was available to you.

1. Sorry, I should have sent the files this morning, but I forgot. I really dropped the ball on that one.
2. You can't rely on Gary. He's always likely to drop the ball when he's put under pressure.
3. Look, I'm not the only one who has been dropping the ball around here - productivity has been down across the board.

1. I should have switched energy provider when our contract was up. I could have saved 200 euro. I dropped the ball there.
2. I completely forgot about my sisters birthday party. I was so busy. I dropped the ball, I'll make it up to her.