You might have been stumped as to why some of or all the catchments from Civil 3D would not export to SSA using the “Edit in Storm and Sanitary Analysis”. You will come across this issue if you have selected the Time of Concentration Method for the Catchment Properties as TR-55 and did not provide the corresponding slopes to the Flow Segments (i.e. slopes for the Sheet Flow, Shallow Concentrated Flow and Channel flow segments) as the default value assigned by Civil 3D is zero slope.
The flow path has no time of concentration as the slope of the flow line is zero. A time of concentration of zero does not export since there is nothing to evaluate. Hence you may want to expand the Catchments from the Prospector tab, right click the Flow Path and select Edit Flow Segments to modify the slopes to a non-zero value.
Another pre-requisite is that, the catchments have to be referenced to a pipe network structure; hence you want to make sure that each catchment is referenced to a structure as shown below: