Obtaining an Entry Visa to visit one of my suppliers in Shanghai area

I am a citizen of Sri Lanka and I am also a permanent resident of United States. Currently, I am working with a German Textile manufacturer in Hochiminh City, Vietnam as Technical Consultant.
I have some suppliers in China that I need to visit from time to time but the Consulate General in Hochiminh City does not issue me an entry for China since I have a Sri Lankan Passport.
1. I do not know what the real reason is!? I know there are so many Sri lankans visiting PRC for business and for pleasure.
2. Please advise me how I can obtain.

Thanks
Sunil Wijesuriya