Not necessarily true

My parents stayed here for a year, went back and stayed in China for about 1.5 years, and came back here again with no problem getting their visa.

I guess it all depends on luck - it is really hard to blame one or two factors. However, I do think preparing and presenting their visa application documents neatly helpful. Here is what I did:

Every time, I prepared all the documents for my parents, and listed clearly in a separate sheet what were included in the application package and that sheet went on top of all the documents so that the visas officer could have a good idea of what they had on their hands easily.

Good luck.