And parents aren't always to blame.
Safety experts at Ohio State University say after looking at 3200 combinations, they concluded many cars and seats just aren't a good fit.
They found back seats vary widely from car to car on the angle, the seat width, and the height of built-in headrests.
To get the best fit, parents need to measure their car before buying a car seat.
"Take your measuring tape in the store with you and measure the one on the shelf or get it out of the box," said Ohio State's Dr. John Bolte. "It's impossible just to sit the car seat on the car, or on the seat pan, and have it match up perfectly."
Parents also say they would love more guidance on what seats work with what cars.
Experts say they're working with both car and seat makers to do that.