In my 3D model I have solids and Forms in a variety of shapes including rectangular, cylinders and cones, each representing various concrete structures. Each pair of a given shape having coincidental edges and all have Parts assigned with a common unifier. However, in my Drawing Model the conical solids and Forms do not unify, even when the unification tolerance is increased.
This issue has been logged as Defect# 330129.
Workaround: Set the BB_UNIFICATION_METHOD configuration variable to the legacy MERGE value which should allow the conical shapes to properly unify. While this is not the default value nor the preferred the unification method; the newer GLUE method is much more robust, this should at least provide an interim solution and allow you to produce the necessary drawings.
Note: Elliptical shaped solids may still be problematic with the MERGE legacy unification method, however, elliptical solids with coincidental edges are a fairly unusual scenario.