Pirat that is because you have not configured a repository for it. For that, you need to add it:
"respositories": [
{
"type": "vcs",
"url": "https://github.com/my/repo"
}
]
Then make sure the package you require is the same name as the composer.json
in the repo and you are specifying dev-master
for the version.
Edit: if it is in Packagist, have your changes been pulled by them and are you specifying the correct version?