My Essix retainer does not cover all of my teeth?

I have had my braces off for more than 2 years and recently my retainer broke. I went back to the orthodontist to get a new one made, and I did not realize until at home that my back molars are not covered by my retainer. I think it was accidentally trimmed too much. I was wondering if I need to get a new one? What could happen? Also, would I be required to pay for a new one since it was more than likely a mistake?