Sometimes, mesh elements are not being included in IFC exports from ArchiCAD. When this happens, it means the IFC file does not contain them and hence they don't appear when imported into Harth.
This can happen for a variety of reasons, such as ArchiCAD templates with different setups or country-specific ones, mean the mesh is not being included by ArchiCAD's generic IFC exporter.
Luckily the fix is easy!
Steps:
Open Options > Element Attributes > Building materials
2. Select the material that you have assigned to your Mesh elements
3. Ensure the box labelled "Participates in Collision Detection" has been checked.
4. Click ok.
Now, when you do a fresh IFC export of your 3D the mesh elements using this material will be included and imported into Harth.