In 2017 Pro I created a non-prismatic but relatively simple volume made up of 4 sided non-parallel shapes, but it wasn't recognised as a solid. All surface were oriented the correct way.

I then broke the surfaces into triangles. Still not recognised as a solid.

Then I deleted the diagonals to get back to the original shape made up of 4 sided non-parallel shapes. Suddenly it was recognised as a solid.

Why. I can't see a logic in this experience.

I have experienced similar behaviour in the past and gave up on trying to create a solid after spending a lot of time trying to edit the shape as I had tried this time.

Is this a bug, and can it be fixed or worked around?

This is the model referred to in the message.

I was hoping to find the intersection of 2 sloped surfaces (a 1:8 and a 2:1) by drawing two objects and rotating the 1:8 sloped object about the vertical axis so that one of its sloping sides snapped on to the 1:2 face.

I cannot work out how to make the line snap to the surface. I can't seem to get a precise intersection.

I can work around by working out the slope distance, and drawing a circle on the 1:2 face of that radius, etc, but that still only gives me an approximate solution becasue of the limited accuracy of the sketchup circles.

Is there a way to make the accurate interesection by rotating one object on to the other?