nobody can exactly answer why they refuse your visa application. Before 2002, my mom came to US twice, my dad once. But they still got visa refusal 3 times from 2003 to 2006. Last month, they finally got visas again in Guanzhou.
I spent many time to write a letter to my parents and also a letter to visa officer(VO). The more important thing is that you must let VO understand your parents will come back to China, and clearly list the reason why your parents will back to China.
Other material is standard. The more, the better. If VO let you go through, he/she may take a look the material. If no, everything you prepared is garbage.
Good luck.
I spent many time to write a letter to my parents and also a letter to visa officer(VO). The more important thing is that you must let VO understand your parents will come back to China, and clearly list the reason why your parents will back to China.
Other material is standard. The more, the better. If VO let you go through, he/she may take a look the material. If no, everything you prepared is garbage.
Good luck.