Import .step file as a Solid

Recieved a bunch of .stp files for components I need in an assembly.
I opened the files to convert them, then saved them as .sldprt files.
I used them in my assembly, and now I want to create some drawings
When I cut a section thru one of the imported parts, it doesn't show
up in the section view. Looking back at the part file, in the feature
tree it only show a surface, not a solidbody.
How do I covert this surface model to a solid, so that I can
manipulate it, and section it in a drawing?
I've received other .stp files and when I open them they come as
solidbodies. Why would this be any different?
Rob