Shouln't there be some jonctions between the external and internal surfaces for SW to "fill" the space?
I'm not really in position to help right now, as I'm not an user of Solid Works, but I'll ask a Technology teacher I know well, he use it daily, I'm sure he'll be able to explain the problem. I hope to cross his way today...
Just to avoid confusion, which version do you use?
EDIT:
Wow, I fear I advanced myself a bit saying it would help. I just saw the teacher and showed him your pic, and although he seemed to understand the problem I will have a hard time explaining in english something I don't fully understand in french.
Basically he said you used a "metal sheet" (~traduction of french word "tôle") effect, which cause the external layer to lack thickness.
He told me you could create an intermediary layer of the appropriate thickness filling the gap, but he think it would be too complicated because the shapes of both layers doesn't match each other.
So in other word his advice is to cancel your external surface, and create a new one, with the basic tools, with the appropriate thickness. If you need the "metal sheet" rendering, create this layer at the end as a third surface. (a bit like applying a texture).
Well, I'm glad if it helped, and sorry if it didn't... That's all I'm worth.
I tried to loft the two surfaces together but that didn´t work either 
I´ll sit tonight and think a bit, if i can´t fix it I guess I´ll have to model the 2 objects separately