I have this same problem, and I’ve seen it in several other places in the forums. But nowhere have I seen a solution. My theme is YGo Horizon, and I have made no changes to the right sidebar. It’s already set up for widgets, but no matter what I do I can’t get an RSS widget to appear in the right sidebar. I select sidebar 2 before I add the widget, but it always goes to sidebar 1. It works okay for some widgets, for example, adding a text widget. But it always defaults to sidebar 1 for RSS. And also for a search box, even though one is already built into sidebar 2. What would be different?
