Why don't newly added files trigger my gulp-watch task?

I have a gulp task which uses gulp-imagemin to compress images. When I add new files to this directory I'd like for this task to compress them as well. I read that gulp.watch doesn't trigger on new files and that I should try gulp-watch so I used it like so;

gulp.task('images', function() {
  watch({glob: './source/images/*'}, function (files) {
    return files
      .pipe(plumber())
      .pipe(imagemin({
        progressive: true,
        interlaced: true
      }))
      .pipe(gulp.dest('./www'));
  });
});

This works the same as gulp.watch on the first run, but when I add a new image to the directory nothing happens. If I overwrite an existing file however, it DOES run the task again, so it does behave differently.

The documentation on gulp-watch called this "Batch Mode" and said I could also run the task on a per-file basis, so I tried this way too;

gulp.task('images', function() {
  gulp.src('./source/images/*')
    .pipe(watch())
    .pipe(plumber())
    .pipe(imagemin({
      progressive: true,
      interlaced: true
    }))
    .pipe(gulp.dest('./www'));
});

But nothing changed. Why isn't adding files to my image directory triggering the task?

Most likely such kind of questions are redirected to gaze package and its internal processes, that runs complicated watching procedures on your OS. In this case you should pass images/**/* to glob option, so gaze will watch all (including new) files in images directory:

var gulp = require('gulp');
var watch = require('gulp-watch');
var imagemin = require('gulp-imagemin');

gulp.task('default', function() {
    watch({glob: 'images/**/*'}, function (files) {
      files.pipe(imagemin({
        progressive: true,
        interlaced: true
      }))
      .pipe(gulp.dest('./www'));
    });
});

But this fill not fix case, when you have empty images directory. If you want to watch them, pass ['images', 'images/**/*'] to glob, and it will watch directory, that initially empty.

P.s. also you dont need gulp-plumber in this case, because watch will rerun function, that uses imagemin every time, even when imagemin pops an error.