Thank you for your response!
The conclusion is very funny.
When I excute
b = Ball[{0, 0, 1}, 1];
p = Prism[{{1, 0, 1}, {0, 0, 0}, {2, 0, 0}, {1, 2, 1}, {0, 2, 0}, {2, 2, 0}}];
t = Graphics3D[{b, p}]
Export["f:\\3.stl", t]
The system tells me "The function DiscretizeGraphics is not implemented for it" and "Graphics3D cannot be exported to the STL format".
When I excute
p = Prism[{{1, 0, 1}, {0, 0, 0}, {2, 0, 0}, {1, 2, 1}, {0, 2, 0}, {2, 2, 0}}];
t = Graphics3D[{p}]
Export["f:\\3.stl", t]
I can successfully ex- and import the prism.
When I excute
b = Ball[{0, 0, 1}, 1];
t = Graphics3D[{b}]
Export["f:\\3.stl", t]
I can successfully ex- and import the ball.
When I excute
c = Cylinder[{{0, 0, 0}, {0, 0, 1}}, 1];
b = Ball[{0, 0, 1}, 1];
t = Graphics3D[{c, b}];
Export["f:\\3.stl", t]
I can only get the cylinder.
When I excute
c = Cylinder[{{0, 0, 0}, {0, 0, 1}}, 1];
p = Prism[{{1, 0, 1}, {0, 0, 0}, {2, 0, 0}, {1, 2, 1}, {0, 2, 0}, {2,
2, 0}}];
t = Graphics3D[{c, p}];
Export["f:\\3.stl", t]
Again, I can only get the cylinder.
I guess if you try your system, you will see the same thing, right? My mathematica version is 10.2.