To check #1 (for the Amiga), pull down The Project editor after you start a render during the initialization phase. Click once on the Workbench backdrop and you should see how much RAM you have on the top of the screen. As Imagine starts to render, this will decrease. If it becomes close to 0, chances are, that's your problem. To solve that, buy more RAM.
The other possibility is that the objects are outside of the world boundary. The world boundary is basically a box in which your objects are placed. When you enter the STAGE editor, you are placing objects in this "virtual box" whose center is 0,0,0.
When you Trace, Imagine clips ALL objects that fall outside of the box. The size of the world boundary is set in the ACTION editor. In this editor, there should be an item named GLOBALS. Whatever numbers are set in the SIZE timeline becomes the size of the box so that it lies from +/- Value for X,Y and Z. The default is no information present, which Imagine assumes is +/- 1024 units for all 3 coordinates.
Note: The world boundary has no effect in Scanline rendering.
Note: Setting the World Size to 0,0,0 regardless of problems will usually DECREASE Trace times!