Hi @OldUser, thank you for your reply!
1) is the address you live at now different to the one you put on I-751?
Yes, we have moved but we updated our physical address and received the address change confirmation (the online account also shows the correct address). Also, we used P.O. Box on our application as mailing address for this purpose just to avoid potential issues with moving.
2) did you successfully receive any other mail from USCIS at current address?
Yes, we have received the mail regarding confirmation of address change of US sponsor in our P.O. Box (mailing address) since the filing of I-751.
3) did you file a case for notice non-delivery? https://egov.uscis.gov/e-request/displayNDNForm.do; jsessionid=08E5FD1FD9BC*****************6F3?sroPageType=ndn&entryPoint=init
You have an advantage, at least you know the case number.
Yes, we did, but didn't hear anything back even after 30 days, so I just filed another one and need to wait another 30 days. we are starting to worry because when we called USCIS back last week the officer said looks like the notice was never generated and there's nothing to send.
4) did you try to get an infopass appointment? If your spouse gets a I-551 stamp in passport, they can renew driver's license and travel internationally for 1 year of its validity.
Yes, we tried to make an appointment through USCIS phone call in October and was given a reference number, we were told someone will contact us within 30 days, however, 30 days have already been passed and when we called USCIS back last week, they said the appointment was never made they are now scheduling another one, hopefully we will hear back within 30 days.
5) does your spouse have an unrestricted social security card?
Yes, I think it's unrestricted since she received it after getting her 2-year green card and it looks identical to mine (just name and SSN on the card).