Solids and quilts behave quite different in Creo. To get a set of surfaces to solidify, you need a closed boundary and merges surfaces into one quilt. A solid may also share the quilt boundary to create the closed volume.
Sometimes it is easier to work with surfaces until you get the shape you want and use either a solidify or thicken to make it a solid object.
Solid objects work better in drawings as well. In general, people working in parametric modelers are after a solid part definition.
