When I import an ASE into Scene Builder and hit Save MTX, nothing appears in the scene when I reopen it. But, when I add primitives, they do appear in the scene. Why?

The reason is that you are not saving the scenes correctly out of Scene Builder. The fact that the primitive is showing up confirms the problem.

There are three buttons for saving your work: Publish, Save MTS, and Save MTX. As you know, when you create a scene, there are two files associated with it: the MTX, a text-based scene description file and the MTS, a binary geometry file that can include internal, compressed textures if you elect to put them there. When you import an ASE, which is also a text file (open it in a text editor and you'll see this), Scene Builder separates the relevant geometry information from the scene description file and creates the two file types- MTX and MTS. However, if you only click on the Save MTX file, only the MTX is created. Thus, your scene is all set to go, but it's missing the MTS geometry information. That's why you don't see anything. In order to save the MTS, you need to use Save MTS or simply click on Publish, which creates both file types in one step.

So what about the primitive? Procedural primitives are named in the MTX and created on the fly by the plugin. Obviously, this alleviates the need for compressing the procedural into the MTS and keeps file sizes smaller.

One more thing, and this is important. You should only save an MTS ONE TIME! If you open up a scene and edit it, and then want to save your changes, you only need to click Save MTX. If you click on Publish, you will be recompressing already compressed geometry, and thus degrading it.