Can't push a snapshot for Apache Nifi to repository.apache.org

I'm a bit puzzled.

I can deploy a staged release for org.apache.nifi, but I can't push a
snapshot, I get permission denied.

Error:

https://repository.apache.org/content/repositories/snapshots/org/apache/nar-maven-plugin/0.0.2-test-incubating-SNAPSHOT/nar-maven-plugin-0.0.2-test-incubating-20150116.131426-1.jar,
ReasonPhrase: Forbidden.

I happen to be using the nexus-staging-plugin for this, and I have
credentials set up in settings.xml.

<plugin>
<groupId>org.sonatype.plugins</groupId>
<artifactId>nexus-staging-maven-plugin</artifactId>
<version>1.6.5</version>
<extensions>true</extensions>
<configuration>
<serverId>repository.apache.org</serverId>
<nexusUrl>https://repository.apache.org/</nexusUrl>
</configuration>
</plugin>

Any clues?