Understanding the concept of what makes an object a Solid will make you a better modeler.
Knowing what to look for makes it easier to fix small problems as you go.
Understanding Solid Objects in SketchUp — Aidan Chopra
A Solid in SketchUp is a single Group or Component whose geometry meets certain conditions.
When those conditions are met, Entity Info indicates the model is a Solid Group or Solid Component.
Also notice Entity Info indicates the Volume of a selected Solid.
• The geometry must form a single airtight vessel*.
Like a perfect soap bubble … •No gaps •No holes •No leaks
• No extraneous Faces inside or outside the vessel.**
All Faces must serve to enclose the singular volume of the vessel.
• No stray Edges.
All Edges must serve to support a Face that in turn serves to enclose the airtight vessel.
Thus, each Edge supports two Faces … no more, no less.
• No nested Groups or Components.
*A Solid Group or Solid Component may contain one or more separate airtight vessels.
**Best that all Faces are oriented Front Side (white) facing out.