I-751 files for month without receipt. XiaoBaiTu & others, pls H

I filed the I-751 (removal of condition on the conditional green card - Married to US citizen and adjusted my status from H-1 to Conditional Green Card) in April and the USPS sent me a receipt that the CA Service Center received my mail on April 6, 2007. I checked my bank and they cashed my check. However, I never receive any receipt from the CA Service Center/USCIS that they receive my paperwork.

I made an Infopass appointment in my local USCIS office on May 1 (as my condiational green card's expiration date was May 4, 2007) mentioning I did not receive any receipt and my GC was expiring. So they gave me the I-551 stamp on my passport which will expire on July 31. They just asked me to wait for a few more week at the time and saying I should receive my receipt. Now, few months has passed and I still haven't receive any receipt. I found the WAC number on the back of my cashed check. But when I put in that WAC number online trying to search my status, it said there's an error (the number was incorrect or so). In my last infopass appointment, the lady also find the same WAC number in the system but she said there was no other information was shown in the computer system. So, she just asked me to wait.

Now, my I-551 stamp will be expiring and I made another infopass appointment on July 27 trying to find out what's going on. I am suspecting if they lost my paperwork?! I thought I sent enough supporting document and even if they have questions, they should have send me a letter to request for more information? (included my son's birth cert copy) Would anyone advice what I should do now and what I should prepare before my next Infopass appointment? My HR also ask me for my work permission paperwork now as my I551 stamp is expiring soon. Do you think I would get another I-551 stamp from my local USCIS office again?

What can I do to follow up with the CA service Center? Please help!! Do I really need to hire a lawer to follow up on this? (I file all my application myself before). Thanks in advance!