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When import a LandXML created from a model, the elements are not correctly imported into Civil3D.
As a test, try to import the LandXML file back into storm and sewer program to see if the model is complete. That can help identify if the issue is with the file or with the program.
Also, make sure you have the latest release and latest update to the product. Once you, try exporting the file into the LandXML format once more to see if that impacts the results.