To copy noncontiguous slides, open Slide Sorter view, click the first slide thumbnail, press and hold ____, click each additional slide thumbnail, release the key, and then click the Copy button.
Based on the information provided within the question, it can be said that you have to press and hold the CTRL key on the keyboard before clicking each additional slide. This would allow you to maintain the first slide as "selected" while also including various other slides as "selected", in order for you to be able to copy all of the "selected" slides when the copy button is pressed.
