How do I find what is preventing a surface from healing?
There are numerous possibilities. Commonly that would be due to the vertices of the polygon surrounding the hole being out of plane. You might need to draw some intermediate lines across the hole. SketchUp sometimes has difficulty with very complex shapes but drawing lines across the shape in several places can convince the face to be healed.
Really from what little you’ve given us, it’s all a guess. If you upload the SKP file so we can see it, we’d be able to give you an exact answer.
What version of SketchUp are you using? Please complete your profile.