This may not apply because my experience has ever been with WebLogic 10, but ...
Make sure that the cluster is configured correctly, because it must be WebLogic (and not Maven or any plug-in), which is responsible for distributing the deployed application to each managed server. The maven plugin should only ever be deployed to the admin server, and WebLogic handles all the other magic to make sure that every managed server that it knows about also gets the application deployed. I do not think the fact that you are using MOJO matters.
Also, make sure that NodeManager is configured correctly and that it is running. NodeManager (at least on WebLogic 10) is responsible for all communication between nodes. The NodeManager process may freeze or kill on a managed server that is not receiving the application. However, it is unlikely that you tried to deploy from the Admin console, which you stated that you tried in your comments on the original question.
Hope this leads you in the right direction.
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