File from Blender is different in Shapeways's preview I am trying to print a model I designed in Blender on Shapeways.

The object has a hole in it:



But when I upload it to Shapeways as a STL file, fixed the sizes, but the hole is filled up in the preview window for the object in Shapeways:



What could cause this? Will the model print properly?

(I cannot share the model here).
Double-check that your model is solid (i.e. watertight). Holes in the mesh, or (as other's mentioned in the comments) or problems with thickness can cause those issues. You can use Netfabb's Cloud Services, or download the free version of their app. There are other model repair services, too.