Even though you had your fingerprints taken, you wont be given an interview until the USCIS recieves a response from the FBI that they have no deragatory information on you. Getting this reponse from the FBI can take anywhere from 1 to 12 months, or sometimes longer. The reason it takes so long is because the FBI does a search based only on your name and birthday and they check for "similar" names and year of birth. The computer may match your name to someone else who has a similar name and close birthday and then an FBI agent must physically look at the files and make sure it is not you. They don't have a lot of people working on this at the FBI and it takes them a long time to get around to it. Basically it is typicall beurocratic red tape. Sooner or later they will send a response to USCIS and then USCIS will invite you for an interview. This is quite typical. You are not the only one. Worry only if you change addresses or you are waiting more than, say, 9 more months.