I was born in China. how to get Chinese visa without my own naturalization certificate

I was born in China. I automatically became an American citizen after my mom’s naturalization in 2003. Now I’m majoring Electrical Engineer in U.S. Airforce Academy and minor in Chinese. I got the opportunity to go to Nanjing University for 3 weeks through our language exchange program. I was so looking forward to visit China again. The academy submitted my visa application through a travel agent( documents including my application form, my new U.S. passport, my expired Chinese passport which was attached to my mother’s expired Chinese passport as a minor). Now I was told the travel agent unable to help me to get my visa, because I don’t have my own naturalization certificate. I told them my US passport is my proof of US citizenship. I didn’t have my own naturalization certificate because I was a minor at that time. Please advise.