Uploading models to GrabCAD. What am I doing wrong?

I've finally got around to uploading a couple of assembly models I've done but the previews of the models won't display. What am I doing wrong? Is there a tutorial or instructions on how to upload one's models?
I can download them and the content is there, it's just the previews aren't showing.
Surely it can't be that difficult. What am I missing? Frustrated.
Accepted answer

Sometimes the viewer takes a long time (hours - days) to generate a preview. Sometimes a preview will never be made (depends on the source program, and version).
Best option is not to rely on the viewer to create a preview. Uploading a simple screenshot puts you in control of the image seen in the library.
Screenshots are good, renderings are usually better.
Also, consider including a neutral format like Step . That allows a greater number of people to use your model. It is also easier for the library to generate a preview image for step files (or at least that has been my experience).
See this upload as an example: Outhouse / Privy - Single Seat
In five months the SOLIDWORKS files have no preview. But who cares? Upload a rendering, and place a step file at the top of the file list so the pictures and the model with a preview show up as the first files on the model page.