importing files from inventor

I used Inventor 10 to model a folk lift carriage. I then derive the part into one solid lump so that I can fill gaps and simulate weld. This way everything is meshed together and there are no unseen gaps to really throw off the analysis. I then send it to another engineer who uses cosmos 2007 (solid works)to do the analysis. We have done this before with no problems.

The latest carriage is now importing as surfaces only. I have saved it as a step file and I have tried not deriving it. All the piece parts can be imported fine alone but the assemblies and weldments become surfaces. I have never used cosmos so i dont know if there are option that need to be looked at. I do know that a similar assembly works fine but this one does not. Any thought would be appreciated.