A few beans short of chili.

A few beers short of a six-pack / a six-pack short of a case.

A few birds shy of a flock.

A few blocks short of a filesystem.

A few bombs/melons short of a full load.

A few bricks short of a wall / hod / load / pile.

A few chips short of a cookie.

A few clowns short of a circus.

A few clues shy of a solution.

A few cold solder joints.

A few ears short of a bushel.

A few feathers short of a whole duck.

A few few cylinders short of a full re-format.

A few fish short of a string.

A few french fries / one hamburger short of a Happy Meal.