gulp not watching <docroot>/themes/custom/<sub-theme>/scss/bootstrap-variables.scss

Any reason that anyone can think of why gulp would not actively watch
<docroot>/themes/custom/<sub-theme>/scss/bootstrap-variables.scss

but if I kill gulp and restart it then it does recompile it?

This is what is in the gulpfile.js but I can't figure out why it wouldn't watch.

const paths = {
  scss: {
    src: 'scss/**/*/*.scss',      ←   watch location
    dest: 'css',
    watch: 'scss/**/*/*.scss'
  },

Thanks