Please check that you're dragging to the correct location in the model.
If you're dragging from the repository explorer rather than the outline view, then make sure that you're dragging from the correct project, as a model can only be dragged into another model belonging to the same project. To be sure of the project to which a model editor belongs, use the editor's right-click -> Show in Repository.
You're also welcome to create one or more screenshots demonstrating your problem and posting them here.
For best results, use the Firefox browser..