Add an new panel, it will set it at the bottom of the table, click on any empty area of the new panel and drag it up or down to the position you want between existing panels and continue your work from there.
If you can’t do that, there might be an issue with your design such as an external panel covering the area or some hidden control that are on the panel moving way. You can create a test PanelsTable control and check this out on it.
