Unable to thicken this surface

Richard,

It appears that you have a curvature issue on the side, near what I would call the back.

It is at the bottom right of the image above.  Us the Evaluate tab and the Radius of Curvature tool (rainbow in a square).

In general, you're trying to do too much in one feature.  In this case, I would create a big rectangular block, then use extruded cuts to get the form, and finally shell the finished form.  You are trying to save time and effort by creating this all at once, but it is better to create simpler features when you can.  You have a form that can easily be created from simple features.  You can build this whole thing from extruded surfaces too, and then trim them together.

The KISS principle applies to CAD too.