Most important point is that your parents will have to prove the ties to Nepal.  The documents you send from here are just secondary evidence.  They might not even look at those.

My father-in-law was denied visa the first time because they said that his ties to Nepal was not sufficient.  Later he reapplied with more substantial documents - employment docs, property, bank balance, etc. - and he was granted visa this time.

At both times, we had sent identical documents.

ps - sometimes I wonder if it is all just crapshoot:  maybe it all depends on what the visa issuing consular feels at that time or which side of the bed s/he wakes up on or whether s/he had a fight with the spouse or whether s/he is constipated, etc.