Iran can no longer pay the Chicoms in dollars, and they can get cheap natural gas anywhere, particularly from the Central Asian countries along the “One Belt, One Road” pathway, without all the headaches the Iran deal brings with it.
Russia is completing a huge gas pipeline to China.
If Iran wants to sell its oil to China, it has to accept payment in Chinese yuan or barter. Even then the companies that do the transport are subject to sanctions as are their banks that pay the crews or other companies that provision the ships. Iran does not own many tankers themselves. The sanctions are hurting Iran badly.