Notify only appears on some gulp tasks

[SKempin]

I am experiencing an issue where the notify only works on certain gulp tasks. For example it works on a SASS to CSS compilation task, but will not work on the simple copy task below. I'm not getting any console errors to assist with this?

gulp.task('pdf-copy', function() {
    return gulp.src(paths.pdf)
        .pipe(gulp.dest(paths.distpdf))
        .pipe(size({
            pretty: true,
            showFiles: true
        }))
        .pipe(notify("PDF task complete"));
});