Check the "DevForce" pane in the output window for any further warnings or messages.
We've recently fixed a few COM problems in the Object Mapper which could occur when saving the model in some scenarios. These fixes will be included in the RC1 release, but obviously won't help you now. If you run into problems, sometimes closing VS and re-starting can "fix" the problem. Also saving and building prior to opening the Object Mapper might help too.
If there are any error or warning messages written out to the DevForce pane, please let us know.