Breakpoints in Python subprocesses

I am running multi-threaded python code inside pytest using the PyCharm debugger.

The test fails, and I get a failure report from the main thread, which catches the result from a sub-process using check_output.  When I try to put breakpoints on the code where the subprocesses fail, though, the debugger does not stop at the breakpoints.

Is there a way to do this? The documentation doesn't give an answer (or I don't know how to find it).