Five months is way too long. Find an experienced immigration attorney to help you.
Mr. Shusterman's (former INS Trial Attorney, 1976-82) response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
I tend to agree that such delay may indicate something amiss in that case or even a possibility that the sent correction was not delivered or even lost. I suggest meeting with a local immigration attorney to assess your further actions to efficiently update the record as necessary to facilitate the case.
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Normally NVC gets their information from CIS which transfers the file to them and rarely they make such mistakes. You need an attorney to review your file and correct the mistake where it happened.