Did you take the exam at the FAA office, or did you take it somewhere else?
Furthermore, is there any major paperwork to be submitted to get the certificate?
If there is a third party involved anywhere in this, you might want to check with them, as they may have forgotten to submit your paperwork.
Funny thing is, this is exactly what happened to me when I was getting my PPL...it was about 15 days before the expiry date of my temporary certificate, so I called the Transport Canada office...they never received the paperwork, so I called my instructor, who very sheepishly called me back 10 minutes later, saying he found the paperwork and was walking it over to the TC
office right away.