It sounds like the dependency didn't download properly. I would try deleting the org.jmock directory from your local M2 repository and let Maven try a fresh download of that jar.
Edit: When you look at M2_REPO\org\jmock\jmock-parent\2.6.0\jmock-parent-2.6.0.pom
in a text editor does it specifically say <packaging>pom</packaging>
? You may want to make sure your dependencies in your Nexus repo are correct as well. I just pulled the dependency from Maven Central and didn't have a problem with it.
Edit 2: It's possible that Nexus will automatically create a parent pom for you like that if it can't download it from Maven Central. Is your Nexus installation configured to download dependencies from Maven Central? Did you manually mvn deploy:deploy-file
your jmock-junit4.jar to your Nexus repo? I'm sure there's a way to force Nexus to update that pom, or you could manually deploy the proper parent pom by downloading it from maven central and then running the mvn deploy
command listed above.