1
general::
Make an error; miss an opportunity. For example, She really dropped the ball when she forgot to call back, or He dropped the ball, turning down their offer. This expression comes from sports where a player who fails to catch a ball is charged with an error. Its use for more general kinds of mistakes dates from about 1950.
American Heritage Idioms
2
general::
Phrase(s): drop the ball
1. Lit. [in a ball game of some type] to let the ball get away or fall out of one’s grasp. • Good grief! Bill dropped the ball, just as he was about to score! 2. Fig. to make a blunder; to fail in some way. • Everything was going fine in the election until my campaign manager dropped the ball. • You can’t trust John to do the job right. He’s always dropping the ball.
McGrawhill's American Idioms And Phrasal Verbs