Call USCIS before response time expires

Sometimes it takes longer over ten days to receive the mail. I would strongly suggest you call USCIS if you have not received it within 10 days, because you are usually given 1 month to respond REF. I have a similar situation, I called USCIS and they promised to resend and fax me REF (Luckily, I also received it today 12 days). Do not wait more than 2 weeks as you do not know how long it will take to prepare response.