父母探親簽證的問題

I want to bring in my parents for a visit.

Things are fine except:
1. We just came back from China in March
2. A few years ago, my parents visited us once and had their visa extended once

Other than these, things are OK.

They are scheduled to interview on 7/5. How likely are they going to get their visa? Is it a good idea to wait till say September?

Thank you very much in advance!