I have discovered another reason why parts may not appear in a model created in Blender/stl.
When you copy parts in Blender and hit Shift+D (Duplicate) once too often, it may happen that you end up with two identical parts in the same position. Such double parts disappear when you upload them to Shapeways.
This is easily solved by removing the offending copies.
It is rather hard to spot, as the model looks completely normal in Blender, but you can check the list of objects to look for duplicate parts.
Greetings
Karl Heinz
PS.Sorry, this should have gone into another topic. I was rather unconcentrated at the moment.
Last edited: Dec 31, 2011