When Parts are added to the Canvas, the order in which they are added is important.
Firstly, parts that sit on top of each other will be displayed in order, e.g. the most recently added Part will appear above the other Part. By changing the ZIndex of parts, you can force Parts to appear below or above each other. Higher ZIndex values make Parts appear above lower Zindex valued Parts.
Secondly, some Parts such as Fitted Components need to be added in the correct order, so other Components can sit on top of them. For example a Switch Component needs to be added first, before a Cover Component can be placed on top of it.
Thirdly, Fitted Component Parts that overlap, will need all but one of the Fitted Components to have their Gestures Enabled set to off, otherwise the Virtual Hardware Builder will not allow the overlapping placement to occur.