Ensure the video & USB cables are connected between each computer and the KVM switch
Safely power off all computers.  Unplug everything from the KVM switch and connect one computer at a time, swapping the inputs previously used (e.g., the computer connected to PC2 now connects to PC1). Power on each computer and confirm functionality before connecting the next one to the KVM switch.
If using a KVM switch that supports multiple displays, ensure all video connections from the computer are from either the video card or motherboard, both cannot be used simultaneously on most systems.