how were you able to find that, I looked for hours trying to figure out what was wrong. I still have not been able to get it to Impose Grid even after I fixed that
In this case I think you are a victim of Sketchup’s inference engine. When you draw the lines Sketchup will snap to the blue axis which will not yield the correct result since the two surfaces have different widths. If you move the top group away along the red axis, it will be easier to connect the groups since the inference engine will not lock to an axis.
Aggressive zoom also helps in overriding inference engine locks…
There is some value in heavier profile edges some times although I prefer to leave them set to 1 for my modeling. Profile edges can help you identify some issues. Turning Profiles off entirely will result in a slightly faster style but that can result in some of the model disappearing. Example: