Pls help: Parents' B2 visa application

Hi,
I am due this Mid-Dec, hoping my parents to come help me. But their visa application in Beijing was just denied and they are giving a second try next week. A main reason, i suspect although none was given, was because of the fact that i got my re-entry visa in Canada last year when i went back to China. i wonder if anyone has similar experience and how i should explain this.
Another question is that: i just got my Green card last month. Should my parents present this fact to the visa officer? Or is it going to have a negative effect on their visa application?

Any response would be appreciated!