python setup.py build suddenly does nothing

Question

I'm debugging a python setup.py file. Stage by stage, I add more sources to it and build, and things are going well, lots of output. All of a sudden, I add two more source files to the list, and suddenly it starts acting as if there were nothing to build. Removing the two source files from the setup.py file doesn't help. Cleaning doesn't help:

C:\Users\Ken\Desktop\Drawbox\Python36 backup> python setup.py clean
running clean

C:\Users\Ken\Desktop\Drawbox\Python36 backup> python setup.py build
running build
running build_ext

C:\Users\Ken\Desktop\Drawbox\Python36 backup>

Is there a known problem here? Is there a way to shake it loose from this misconception? Here's the setup file:

from distutils.core import setup, Extension

drawbox_module = Extension(
    "drawbox", 
    include_dirs = [r'..\Drawbox Library', r'..\Drawbox Library\Win', r'..\Drawbox Tools', r'..\AP Classes'],
    define_macros = [('_ALLOW_KEYWORD_MACROS', 1)],
    extra_compile_args = '/W0',  # silence all warnings
    # sources = ["PyDrawbox.cpp", 'Drawbox.Win.cpp'])
    sources = [r'..\AP Classes\apstring.cpp', r'..\Drawbox Library\Color.cpp', r'..\Drawbox Library\Point.cpp', r'..\Drawbox Library\PVector.cpp', '..\Drawbox Library\Rect.cpp', '..\Drawbox Library\SpaceCalc.cpp', '..\Drawbox Tools\TextUtil.cpp', '..\Drawbox Library\Timer.cpp']
)

setup(name="drawbox", version="1.0", ext_modules= [drawbox_module])

System: Windows 7 SP1, Python 3.6.2, VS 14 command line tools (Visual Studio 2015 Express)