permissions - CHMOD to make directory writable by group not working

I'm trying to make the node_modules directory writable by www-group, but chmod isn't working. vagrant (user) is part of the www-data group as seen with the groups command.

Why can't I permit www-data (group) write permissions to this directory?

vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app$ groups
vagrant www-data


vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app$ ls -l
total 16
drwxr-xr-x 1 www-data www-data  136 Nov 27 22:56 apis

drwxr-xr-x 1 www-data www-data  170 Oct  6 23:24 assets
drwxr-xr-x 1 www-data www-data  170 Jan 15 19:38 controllers
drwxr-xr-x 1 www-data www-data  170 Jan 16 23:00 core
-rw-rw-r-- 1 www-data www-data 1300 Jan 16 23:09 favicon.ico
-rw-rw-r-- 1 www-data www-data 2602 Jan 17 20:03 Gruntfile.js
-rw-rw-r-- 1 www-data www-data 1352 Jan 16 16:27 index.php
drwxr-xr-x 1 www-data www-data  170 Jan 15 19:36 models
drwxr-xr-x 1 www-data www-data  578 Jan 17 03:00 node_modules
-rw-rw-r-- 1 www-data www-data  472 Jan 16 21:11 package.json
drwxr-xr-x 1 www-data www-data  204 Jan 17 01:35 views



vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app$ sudo chmod g+w node_modules --verbose
mode of ‘node_modules’ changed from 0755 (rwxr-xr-x) to 0775 (rwxrwxr-x)


vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app$ ls -l
total 16
drwxr-xr-x 1 www-data www-data  136 Nov 27 22:56 apis
drwxr-xr-x 1 www-data www-data  170 Oct  6 23:24 assets

drwxr-xr-x 1 www-data www-data  170 Jan 15 19:38 controllers
drwxr-xr-x 1 www-data www-data  170 Jan 16 23:00 core
-rw-rw-r-- 1 www-data www-data 1300 Jan 16 23:09 favicon.ico
-rw-rw-r-- 1 www-data www-data 2602 Jan 17 20:03 Gruntfile.js
-rw-rw-r-- 1 www-data www-data 1352 Jan 16 16:27 index.php
drwxr-xr-x 1 www-data www-data  170 Jan 15 19:36 models
drwxr-xr-x 1 www-data www-data  578 Jan 17 03:00 node_modules
-rw-rw-r-- 1 www-data www-data  472 Jan 16 21:11 package.json
drwxr-xr-x 1 www-data www-data  204 Jan 17 01:35 views




vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app$ cd node_modules && mkdir test
mkdir: cannot create directory ‘test’: Permission denied

I also tried using the -R (recursive) flag on the CHMOD command which didn't help, I still can't write to directories; however, I can write to files as the group has write access. Why isn't this working for directories?

vagrant@vagrant-ubuntu-trusty-32:/var/www/src/my_app/node_modules/grunt$ ls -l
total 20

-rw-rw-r-- 1 www-data www-data 1242 May  9  2014 appveyor.yml
-rw-rw-r-- 1 www-data www-data  127 May  6  2014 CONTRIBUTING.md
drwxr-xr-x 1 www-data www-data  136 Oct  2 12:41 internal-tasks
drwxr-xr-x 1 www-data www-data  170 Oct  2 12:41 lib
-rw-rw-r-- 1 www-data www-data 1062 May  6  2014 LICENSE-MIT
drwxr-xr-x 1 www-data www-data 1020 Nov 20 20:29 node_modules
-rw-rw-r-- 1 www-data www-data 2319 Nov 20 20:32 package.json
-rw-rw-r-- 1 www-data www-data  900 May  6  2014 README.md