Just a reminder, questions of this nature should be in this forum:
http://forums.autodesk.com/t5/autocad-civil-3d-general/bd-p/66
This stormwater forum is primarily for questions related to SSA (Storm and Sanitary Analysis) and the Hydraflow programs.
Now back to your question. You need to look at the minimum vertical height of the structure you are using, perhaps use one that has no grate. Try changing to a different structure to see if this works for you. The structure you are trying to use probably cannot be made that short.
Bill