I am modelling a simple treasure chest, shown in the screenshot below. However, I am having issues with loop cuts not cutting through the whole model.
As you can see, the yellow line is the loop cut but I would expect it to follow the red line drawn on the screenshot (excuse the rough drawing).
After a Google search, I tried selecting Non Manifold geometry and this highlighted a few of my edges. I think I understand the concept of Non Manifold geometry, but I don't understand what is wrong with my model or how to get around it.
The blend file is attached.