Visitor Visa - B2

The first time my parents went for interview, the consulate told them that they need to bring more documents regarding my current status in the U.S. At the time I was still waiting for greencard, so I sent them all the letters I received from INS to prove that my greencard is pending. And the first time, my husband wrote the invitation letter saying that he invites my parents to come to the U.S to visit his family and our new born baby.

The second time when they went back for interview, I wrote a invitation letter to my parents saying that I would like them to visit us and my husband's family.
here are the questions they asked my parents:
Q1. how many children do u have?
Answer: just one child
Q2. How long has your daughter lived in the U.S?
Answer: 10 years
Q3. How much do you make?
Answer: $2000 yuan
Q4. What does your daugher do?
Answer: health insurance.
Q5. What's the purpose of your trip to the U.S?
Answer: to visit my daughter, son-in-law and his family.
Q6. Why the purpose of the trip in the second invitation letter is different than the first invitation letter.

Then their visa got rejected because insufficent evidence to prove that they are going back to China.

Can anyone give me some suggestions that what should my parents do? Shall I or my husband write invitation letter to my parents?

Thank u.