The input shortcut isn't defined or DDM isn't installed on the active controlling PC.
Define the shortcut. Open DDM. Go to the Input Manager tab. You can define a shortcut key to quickly switch to a favorite input and another to toggle between two inputs. Save changes.
Install DDM on the PC you use to control switching. DDM communicates with the monitor even when it's displaying video from another computer. Install it on the PC you use most often for control.
If that didn't work: Check the physical USB upstream connection. For KVM functionality between PCs, you must assign both the Video Input Port and the USB Upstream Port for each PC (PC1-PC3) in the KVM setup interface.
Heads up: Multi-Monitor Sync (MMS) limits control. If MMS is ON, DDM has limited control over monitors in a daisy-chain beyond the first one. This can affect input switching.