I have a large and fairly detailed assembly with various sub-assemblies, which in turn have various sub-assemblies each. I have tried both options for STEP output, but both yield the same result; It takes a while to think about it, and eventually outputs a file. When I open this file to check it, it thinks about it again for some time, and then gives me a nice assembly of about 6 parts, the rest are missing.

I tried saving as IGES to test and although a number of errors, I got a working model, but my customer has requested STEP.

Any thoughts? I doubt I can upload model due to copyright (t's a full product) and also due to size.