Why weren't certain Python files examined by coverage.py?

Question

I'm using coverage.py to measure test coverage in a Python project, but some Python files are not showing up in the coverage report. One thing that is different about these files is that they were not executed by any tests (i.e., if they were included in the report, their coverage would be 0%).

What are possible reasons why these files were not examined? If it's because they were never loaded during unit testing, is it possible to force coverage.py to include these files in the report?