3-sided surfaces are always a pain. Even when you divide one side into two. Your problem area is where Sketch10 meets Sketch17.
If I trim away just 1/150000 in it will offset. Doing this you will also see that the offset surface is divided into two surfaces. This is often a danger sign.
Another way to create you surfaces is using Filled Surface instead of Boundary Surface. I know the geometry is slightly different and you have less control, but it could save your day.
Finally sometimes Loft Surface produces a better result regarding boundary conditions though I generally prefer Boundary Surface. And in this case Loft Surface does make an offsettable surface.