Why doesn't composer pull nested dependencies?

Question

git

Working with yii2 and learning how to create packages.
I created my package and registered it in the main configuration file:

"require": {
        "php": ">=5.4.0",
        "yiisoft/yii2": "*",
         ...
        "nepster-web/yii2-module-users": "dev-master"
    },

composer update:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Installation request for nepster-web/yii2-module-users dev-master -> satis
fiable by nepster-web/yii2-module-users[dev-master].
    - nepster-web/yii2-module-users dev-master requires nepster-web/yii2-modules
-installer dev-master -> no matching package found.

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your min
imum-stability setting
   see <https://groups.google.com/d/topic/composer-dev/_g3ASeIFlrc/discussion> f
or more details.

Read <http://getcomposer.org/doc/articles/troubleshooting.md> for further common
 problems.

The thing is, my yii2-module-users package depends on 1 more yii2-modules-installer package .
In the composer.json configuration file of my yii2-module-users package, I have specified all the dependencies:

"require": {
        "yiisoft/yii2": "*",
        "yiisoft/yii2-swiftmailer": "*",
        "nepster-web/yii2-modules-installer": "dev-master",
        "vova07/yii2-console-runner-extension":  "dev-master"
    },

And while installing I run into the above error.
However, if you specify the required package in the main composer.json file:

"require": {
        "php": ">=5.4.0",
        "yiisoft/yii2": "*",
         ...
        "nepster-web/yii2-modules-installer": "dev-master",
        "nepster-web/yii2-module-users": "dev-master"
    },

That everything works as it should.
Can you please tell me why composer does not pick up all the dependencies of the yii2-module-users package ?