When you use hierarchy in design, you add subcircuits into your current design. Ctrl + K is the short cut to add a subcircuit to your current design. In a large design, this list can be long in the add subcircuit dialog. In such instances, it may be easier to find the schematic you want to add as a subcircuit in the project tree.
To do this, first right click and drag on the schematic to add to a different schematic.
Continue to drag the mouse over the open schematic that will contain the new subcircuit.
When you release the mouse, you will get options. Choose the Add as Subcircuit option.
Finally, the subcircuit will be added to your schematic.