I would guess that it is transparent or has a transparent fill color. Others have complained about this in report an issue.
You may want to do a "lasso drag" to select it, give it a visible fill color, drag it and then change the fill color back to transparent. You can also use the zoom controls to zoom way in before clicking on what will now be a larger rectangle border.
Either that or use other methods to move it to where it needs to be such as using the arrow keys or the alignment tools in the Inspector's Position tab.