Velocity results show as zero for a conduit set as a Diversion.
When modeling a diversion in a conduit, diverted flow (based on the rating table) is passed on to the next downstream element. The GVF-Rational (StormCAD) numerical solver approximates the hydraulic grades (and thus velocities) through a profile of a diversion link with a Gradually Varied Flow backwater calculation based on the downstream HGL. Because of this, the "Velocity" result (which is the average velocity) may not be applicable. Instead, you would refer to both the "Velocity (in)" and "Velocity (out)".
An alternative option would be to change the average velocity method to "simple average" and then the "velocity" result will show the average between the velocity in and velocity out.
Modeling a flow split (diversion) with the SewerCAD or StormCAD numerical solver
Velocity Calculations and the Average Velocity Methods in SewerCAD and StormCAD