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When importing a LandXML file created from a model StormCAD, SewerGEMS, CivilStorm, or SewerCAD, the elements are not correctly imported into Civil3D.
As a test, try to import the LandXML file back into the OpenFlows 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 version of the program, then try exporting the file into the LandXML format once more to see if that impacts the results. Furthermore, see the information in the below links for more on the LandXML import/export workflow
Exporting model file into a format readable by AutoCAD Civil3D or Land Desktop
Setting up a parts library in Civil3D for LandXML import
Using a Pipe Network from AutoCAD Civil 3D in StormCAD