Barry Rubin explains here why China won't agree to sanctions against Iran:
"Chinese state companies are currently developing the following oil projects in Iran: Kouhdasht; Yadavaran; North Pars gas field; North Azadegan oil field; Masjed Soleyman oil field; Garmsar block exploration and development. Most of these are still in development but are vital for the future of Chinese development. Total investment: well over $6 billion. And there’s a lot more Chinese involvement in Iran’s economy, including construction of a huge oil refinery.
The Chinese want to make money and ensure their energy supplies. Sanctions on Iran run counter to both these goals.