For a model to be “live” or accessible, it must be deployed, this can be achieved from the model edit screen (i.e. you've opened the model in question), selecting the cog icon on the toolbar, and selecting deploy model.
Once a model has been deployed, it may be adjusted, and the effects will be effective immediately, without needing to re-deploy that model.
If a model is moved to another machine / instance, then the newly imported model (even if was previously deployed), will need to be deployed against on the new machine / instance.