The most common reason an STL file may be not quite right is a mechanical connection where some piece isn’t quite as the designer expected, so the 3D model that interfaces with it no longer fits.

3Shape presently provides STL CAD design file export from its design software. Open STL CAD design file export means professionals can manufacture their designs on any mill and 3D printer.

What a let down. Part of this is due to shortcomings in the STL file format. There’s a newer format available, 3MF, and Josef Prusa and Jakub Kočí would like you to start using it.