Do you know, that… ?

If the deposit or part of the components are transparent, you can not select transparent walls, if it is under a different geometry.

In contrast, despite wanting to mark the wall below the part, hold down the SHIFT key.

The same applies to individual parts. If there are transparent walls and want to show them again in full color – you have to select it and turn off Change transparency.

. Here also comes with the help of SHIFT, permitting to select a transparent wall in spite of the underlying geometry.