3D Printing: File from Blender is different in Shapeways's preview (2 Solutions!!)

3D Printing: File from Blender is different in Shapeways's preview
The Question: 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).

Solutions: Please watch the whole video to see all solutions, in order of how many people found them helpful

== This solution helped 2 people ==
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.azurewebsites.net/, or download the
free version of their app. There are other model repair services, too.

With thanks & praise to God, and with thanks to the many people who have made this project possible! | Content (except music & images) licensed under cc by-sa 3.0 | Music: www.bensound.com/royalty-free-music | Images: stocksnap.io/license & others | With thanks to user reemaj (askubuntu.com/users/4257), user MicroMachine (askubuntu.com/users/4099), user Memnarch1113 (askubuntu.com/users/4241), and the Stack Exchange Network (3dprinting.stackexchange.com/questions/2737). Trademarks are property of their respective owners. Disclaimer: All information is provided "AS IS" without warranty of any kind. You are responsible for your own actions. Please contact me if anything is amiss at Roel D.OT VandePaar A.T gmail.com.

Видео 3D Printing: File from Blender is different in Shapeways's preview (2 Solutions!!) канала Roel Van de Paar