父亲绿卡面试通知问题请教!

我在USCIS网上查到 I-485 application case status：

As of August 18, 2017, we are ready to schedule your Form I-485, Application to Register Permanent Residence or to Adjust Status, Receipt Number MSCXXXX, for an interview. We will schedule your interview and send you a notice. Please follow any instructions in the notice. If you move, go to www.uscis.gov/addresschange to give us your new mailing address.

8/18到今天都三个多星期了，还没收到面试通知的notice。请问这样正常吗？我怕信寄丢了。我是应该再等等，还是要去问一下？如果问去哪里问？

 Thanks in advance!