Muptiple visa entry for one year to PRC

I am sourcing and import export agent cum importer. I need to visit china frequently as per my need for inspection, new product company verification and customers help. Having many suppliers for various products, I requested for multiple visa but the concerned office refused and ask for letter from chinese foreign affairs department. I am unable to understand how to get this letter. My suppliers are ready to help in this regard. I am visiting since 2004 and having business relations with so many companies.