Recalling vehicles is tough enough.But General Motors admits finding the car owners is also difficult.

In fact, nearly 140-thousand recall notices to GM customers so far could not be delivered.

That's about six-and-a-half percent of people who were sent notices as a result of the automaker's ignition switch recall. Officials with General Motors say the primary reason these notices were undeliverable is because it's tough to find the owners of older vehicles that may have been sold a couple of times. GM says they'll keep trying though because they are hoping for 100-percent response to the recall.

GM has suffered 66 recalls in the U.S. this year alone.