It is impossible to guess what is causing this delay which is by far much longer then usual. I agree with my colleague that you should make an INFOPASS for an update. It may be simply the file was reassigned to a new agent or they have had difficulty locating records from other jurisdictions. Good luck.
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Schedule an InfoPass appointment, you will be told the same thing.. The only way, if you feel you have a clean and approvable case, is to have that same lawyer file a mandamus action against USCIS, compelling it to adjudicate it forthwith.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
Your lawyer should write to USCIa with a draft of a petition for writ of mandamus to show them you mean business. Give USCIS a deadline and if they do not adjudicate your N-400, then be prepared to file the mandamus action in US District Court. If successful there a federal judge may also require the government to reimburse you for the attorney fees you incurred.