I was in the same situation, please see below my timeline. I couldn't get a stamp after completing my interview. So I wrote to my congressman (reply said it's standard back log blah blah blah), to FBI NCPP director (never a reply), send email to FBI email every week -- every Monday morning I get to work, my first thing is to forward my previous email to FBI, for 6 months, never a reply. I called FBI office, never got an alive person. So in the end I was so fed up, I read somewhere that I can sue USCIS. Then I called my lawyer, he told me not to do it, and just be patient. I could not accept that as an answer. So with the last hope, I sent FBI a request of information under FOIA (freedom of information act). Apparently they use the same database as NCPP name check for immigration. But for immgiration they go one step further, i.e., they'll check anything involves you, even if you are a victim or witness... so after I sent that request to FBI in mid April 2006 (don't fax, cause they don't accept fax now, even if their form says you can fax), miraculously my name check was complete, and my GC showed up in less than 3 weeks. So it's still a myth to me as to what really helped, but I could only say FOIA won't hurt. Go to FBI website and type in FOIA or just google it, you can find the form and fill it out and mail it in. GOOD LUCK!!!
