ADDING NEW INSTRUMENT

When I add another instrument to an existing instrument, the new instrument will replace the existing one. I have to click on the empty space below the instrument first, then select a new instrument and it will add it, not replace. I thought this was fixed, in loading options I have merge new multi with existing one checked, and have files, libraries, Quick load: double clicking a file replaces currently selected instrument slot unchecked. This is on Kontakt 7.10.7. Does anyone know of a way to add an instrument without replacing the existing one without having to click on the empty space first? Thank you.