Selecting segments from three video tracks to use in picture-in-picture

HELP! I have recorded a subject from four different angles, and thus I have four video tracks. One of the four tracks will function as the permanent backdrop. I am attempting to select random segments from the other three tracks for use as picture-in-picture. I am able to place the first PIP but durring playback, the PIP clip becomes the backdrop video. I can't seem to find a workaround. I'm using Elements 2020 BTW.

Thanks for any suggestions.