Well, I just opened the file used <File <Export <2d and all elements exported to the 2d file. What version of Microstation are you using?
Something you can try if you are still having an issue is to copy the file, set everything to elevation 0 and then export that file to 2d. It's not a solution but at least a workaround until you figure out what is causing your problem. I would contact the Support Group.
