221g from given requesting client letter, but client refused to provide a letter


I Got 221g blue slip on 3rd March 2014 at Chennai consulate on my H1B petition. The VO asked me many questions regarding my employer, client, pay and she finally asked Client letter which I did not have. Then she gave me 221g with bunch of documents ticked to be given by my petitioner. One of them was client letter, which my client refused to provide until I travel and finish all security procedures in USA. How this issue can be solved? Another client is offering me a job onsite, so my employer is thinking to change my client. Is this possible. What is the procedure to change client after attempting an interview?