Why is there no picture on my monitor through the HDMI port on the Pro Hub Slim?

If your monitor shows no signal through the HDMI port on the Pro Hub Slim (ST-HUCPHSM, ST-HUCPHSS, ST-HUCPHSD), check the following.

How to fix a blank monitor on the Pro Hub Slim

Confirm your computer's USB-C port supports video. The Pro Hub Slim sends video using DP Alt Mode, the DisplayPort signal carried over USB-C. A data-only or phone charging port will not output video.
On Apple Silicon M1 and M2 base MacBooks, only one external display is supported, so a second screen will not appear.
Reseat the HDMI cable at both ends, and try a different HDMI cable rated for 4K.
Lower the monitor's resolution or refresh rate if the screen flickers or drops out.
If your laptop has a thick case, remove the case so the Pro Hub Slim seats fully in the port.