STEP models have multiple ways to attach representations to products.
One way is used to describe alternative representations (i.e. solid and sheet representations of the same geometry), another is used to describe complementary representations (i.e. hybrid solid-sheet part).
Some files are exceptions to this dichotomy - they use an "alternative" mechanism to store complementary representations. In such cases CAD Exchanger can appear to read only a portion of the model.
STEP_ReaderParameters has a parameter named
BRepRepresentationType. It is there precisely for this situation.
Its default value is "Auto", so it loads "the highest quality" representation (which is usually solid) by default. This is fine for the majority of STEP models.
However it can be set to "All" to force the reader to load everything, aa if all the bodies were complementary. If there are some parts missing in your STEP model after import, try setting BRepRepresentationType to All.