It seems your parents will need to demonstrate stronger ties to Nepal.  The documents you send are secondary to this factor.  The fact that all of their children are in the US did not help either.  So, I suggest you ask your parents to provide proof of strong ties, such as:

• Job verification letter - with short term (a month long) vacation permission to visit the USA
• Property ownership - more the better
• Business ties - more the better
• Recommendation letter from your father's (and/or mother's) boss stating that he will return to his job in Nepal

There could be more, these 4 are all I can think of.

I was in a similar situation, but after careful evaluation of the said documents, the matter was resolved.

Hope this helps.