Connecting Mac Pro (2012) and Mac Mini (2024) to Samsung Monitor with VGA switch

I have an older Mac Pro desktop OS 10.12.6 and a new Mac Mini OS 15.1 that I would like to connect to a Samsung C27F591 monitor with a VGA switch. I have tried 3 different switches with no luck; either one Mac shows or nothing at all.

Am I connecting the cables incorrectly? I am at a loss; my VGA cables work fine when I use them directly (without the switch).

Any help at all would be greatly appreciated!