Grunt less won't replace css file - gruntjs

Question

I use grunt-contrib-less with grunt-contrib-watch to compile my less files automatically upon change.
When the css is not present, grunt compiles it ok. When a css already exists and watch sees the less file changed, the css file is not modified. I have to remove it every time and let grunt recreate it with the modifications.
less config:
less: {
options: {
banner: '<%= meta.banner %>'
},
dev: {
options: {
sourceMap: true,
sourceMapFileInline: true,
compress: true
},
src: '<%= meta.dev.less %>/main.less',
dest: '<%= meta.prod.css %>/main.css'
},
prod: {
options: {
plugins: [
new( require( 'less-plugin-clean-css' ) )( {
'advanced': true,
'compatibility': 'ie9'
} )
],
},
src: '<%= meta.dev.less %>/main.less',
dest: '<%= meta.prod.css %>/main.css'
}
},
I'm under Windows 10 and every user have the rights to modifiy/delete the files in my dist folder.
How can I let grunt modify the css file?
EDIT
watch config
watch: {
options: {
livereload: 6325
},
js: {
files: [ '<%= meta.dev.js %>/main.js', '<%= meta.dev.js %>/plugins/*.js' ],
tasks: [ 'newer:concat' ]
},
images: {
files: '<%= meta.dev.img %>/**/*.{png,jpg,gif,svg}',
tasks: [ 'newer:imagemin' ]
},
css: {
files: '<%= meta.dev.less %>/**/*.less',
tasks: [ 'newer:less:dev' ]
}
}
Registration
grunt.registerTask( 'default', [ 'less:dev', 'concat', 'imagemin', 'copy', 'watch' ] );
Grunt output (verbose)
>> File "dev\less\elements\menu.less" changed.
Initializing
Command-line options: --verbose
Reading "Gruntfile.js" Gruntfile...OK
Registering Gruntfile tasks.
Reading package.json...OK
Parsing package.json...OK
Initializing config...OK
Loading "Gruntfile.js" tasks...OK
+ default, prod
Running tasks: newer:less:dev
Loading "grunt-newer" plugin
Registering "D:\[...]\static\node_modules\grunt-newer\tasks" tasks.
Loading "newer.js" tasks...OK
+ any-newer, newer, newer-clean, newer-postrun
Running "newer:less:dev" (newer) task
Options: cache="D:\\[...]\\static\\node_modules\\grunt-newer\\.cache", override=undefined
Files: dev/less/main.less -> dist/css/main.css
No newer files to process.

Answer 1

Here's what happens:
you modify menu.less
watch detects that and runs newer:less:dev
less:dev uses file main.less as only source (not menu.less)
that file hasn't changed, so newer concludes it doesn't need to run the task again
I suppose main.less includes menu.less, but newer doesn't know it.
So my suggested fix would be to get rid of the newer part.

Answer 2

Have you added correct watch config for less ?
watch: {
less: {
files: ['less/**/*.less'], // which files to watch
tasks: ['less'],
options: {
nospawn: true
}
}
}
To run grunt with your profile : grunt.registerTask('myprofile', ['less:dev', 'watch']);