父母探亲被拒,请问如何向参议员写申诉信

I am currently in NY and my parents applied for visa in Chengdu (my dad had been on business trips to US multiple times). The visa officer collected their passports and asked them to wait for notice. One month later, they notified him to pick up the passport but didn't give visas. I was really disappointed and want to complain to senator on this (heard this might help).

Has anybody have the similar experience? Do you know how and where to send this complaint letter?

Thanks a lot in advance.