When you have created locks for a specific layout, you cannot add more locks to the layout. Selecting the layout from the layout options only adds the created locks to the list, allowing you to edit them. The only exception to this is custom layout, where preview locks can be created even when created custom locks already exist.
The following example illustrates the way selecting different layouts is handled.
Each time you return to this layout, these created locks are displayed and can be edited. However, you cannot add any further locks to this layout.
Selecting the second layout will display the preview locks again.