If you're trying to use a network license and not having success checking one out, it can be for 2 reasons. The first is that the computer is not set up correctly to access the network license. In this case, follow the instructions in this tutorial to make sure its configured correctly. The second reason could be that the network license server is down, or connectivity to the server has been cut. This can be verified in this article.
Once you're connected to the license server WebAdmin page (http://<your-server-address>:22350), If you can see the landing page, navigate to the Licensing/All licenses tab
This will list the number of licenses and the number of licenses available.
If there are no available licenses for Muk3D or Ooze, then you'll be unable to launch Muk3D until another user has closed their software.
To see the users. you can navigate to License monitoring/Sessions.
It is also possible to verify who is using the server in Catapult by running Licensing/Show connected licenses.