Adjustment of Status - I-485

hi everyone, i applied for adjustment of status for both my parents while they were visiting me in america last year in july. their biometrics were done within a month. Its been over nine months now and we have not received approval notice or an interview date yet nor have they asked for any additional documents. i called the uscis and the agent told me that the field office where our application went is still processing the files received on or before july 4, 2016. initially i was glad because our application was received on july 19 , 2016. i was waiting for the good news but when i checked the field office site for updates 10 days later they changed the dates to june 18 . they went back instead of going ahead. And the most weird thing is that 2 similar cases in my known circle were approved and issued green cards within 6 months in my same state of Michigan. wonder which field office their cases were processed ? i don't know what to do now other than waiting.

            If someone have any idea please enlighten me . THANX