This is what I would do:
- double check if the wall has a fill attached
- double check if you have "vectorial hatching" in display settings
- if you can actually see thru the wall it means either it is transparent or corrupted (sometimes bad library part as window or door can corrupt whole wall - check that too)
- select the wall, open dialog window, click OK - it will refresh it parameters
- redraw section line, try again
HTH, Cheers
Edit:
I don't need to tell you that in Section Tool Settings -> Model Effects there is option "Vectorial 3D Hatching" that need to be checked, do I?