Some ProStructures information is missing from the i-Model created by using Bentley's i-Model Composer
You need to download the ProStructures Plug-in for Bentley products (08.11.14.223) and then create an i-Model. The new i-Model will contain all ProStructures information associated in the parent DGN file.
ProStructures Plug-in for Bentley products (08.11.14.223)