You can only ever have one detail template in a model - so if you have already defined one detail, the option will be greyed out. You might have confused Monarch by creating and deleting templates without saving the model, but I've never seen that behaviour before.
Actually I did delete the detail template but it still did not give me the option of creating a new one. I acually ended up opening up another model and editing it to fit the report. Then I saved it as the new model.