As you might know, Nexus is our (GlassFish) repository manager. It so happens that it is down sometimes. If that happens, you get some download errors. Most of the time, you don't need Nexus dependencies as they are available in your local repo.
But if the build is failing due to Nexus server is down, use the following options to bypass Nexus:
1. If you're outside of Sun's Firewall (SWAN), use "-P external-repo" profile
mvn -P external-repo clean install
mvn -P internal-repo clean install
mvn -DskipTests=true -o clean install