"I wanna share my experience. My parents applied in Nov 2009 for my Dec Graduation but denied the visa. Counselor (She) even didn't look at the documents. She asked some family questions such as how many kids you have? What are they doing?--- Finally She said "Tapai aayogya hunuhuncha." Dad applied second time and again denied the visa. Reason was nothing. Among three, two of my siblings are in US. so,  I think counselor may thought, they won't return back. They showed enough bank balance as they are involving in business but still denied the visa" I guess it depends on luck and counselor mood."

But,
 your family situation is different than mine so, your parents should apply again. They still have chance to get visa. I agree with OC101 comment too.