Python-mode's linters not working on 4 vim installations

Question

Too All,

I cannot get python-mode's linters to work on 4 vim installations.

On 2 PC's running Windows 10, and Python 2.7.14, I upgraded from vim 7.4 to 8.1. Steps as follows for vim 8.1:

mkdir C:\Users\Steve\vimfiles\pack\plugins\opt
cd C:\Users\Steve\vimfiles\pack\plugins\opt
git clone https://github.com/python-mode/python-mode
cd pymode
git submodule update --init --recursive
:packadd python-mode
:helptags ~/vimfiles/pack/plugins/opt/python-mode

my _vimrc and .vimrc (see Ubuntu installations below) for several years have this:

" pymode - Note:  see :help Pymode for defaults already set
let g:pymode_trim_whitespaces = 1          " trim unused white spaces on save
let g:pymode_indent = 1                       " PEP8-compatible python indent
let g:pymode_doc = 1                      " turns on the documentation script
let g:pymode_lint_on_write = 1 " check code on save if file has been modified
let g:pymode_quickfix_maxheight = 12
let g:pymode_lint_signs = 0             " don't show left error symbol column
let g:pymode_lint_checkers = ['pylint', 'pyflakes', 'pep8', 'mccabe']

In Windows 10, when I test the linters with deliberate coding errors, nothing happens.

On the 2 PC's running Windows 10, I have the Ubuntu subsystem installed which includes Python 2.7.12, Python 3.5.2, and vim 7.4. I use Vundle to manage my plugins. Steps as follows for vim 7.4 with Vundle:

:PluginClean   (to remove old python-mode)
:PluginInstall    (to install new python-mode)
:q
cd .vim/bundle/python-mode
git submodule update --init --recursive

In Ubuntu, when I test the linters with deliberate coding errors, the 1st error is reported and then the linters quit.

I have used python-mode's linters for several years on several different computers. Either something is broken in this version of python-mode, or I did something wrong in 4 python-mode installations. Please help.

Thank you,

Steve