It just means that they are being slow. If they have acknowledged receipt but just not sent anything yet, then there is no indication of a problem. If you receive some kind of denial or request for evidence, then you may have a problem.
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Nothing to worry about (yet). They are just being slow. Best to schedule an INFOPASS appointment at your local district office and ask what is happening with this case.
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Just wait out the 30 days as you were told, and if you haven't heard anything by then call again or schedule an infopass appointment with your local USCIS office. The internal delay is just that, an internal delay but the inquiry should hopefully speed things up.
Attorney Jennifer Ahn's answer is to provide general information only, and should not be construed as legal advice on any subject matter. The content of this answer is not intended to substitute accurate legal advice, which is relevant to all the facts and circumstances of your specific case. This answer does not create an Attorney-Client relationship.