MBP in Clamshell mode: display doesn't come on when waking up

I have an M1 Macbook Pro (2020) running Ventura. Most of the time I use it in clamshell mode. I can easily wake it from sleep the using the external wireless keyboard (I hear the fans come on, the external drive flashes, there is a 'dong' sound when I press Option+Cmd - so the mac is definitely awake). But very often the display just says no signal and goes to sleep. It is very inconvenient to take the mbp out of it's stand and open up the display to log in - which does turn the external monitor on. The only trick that works is to unplug the power lead from the mac for several seconds and then plug it back in. The technical details of my set up are: 1. The mac is connected to an Anker USB C hub. 2. The hub connects via HDMI to an HDMI switch (which also has an input for a Windows laptop where things work fine). 3. The HDMI switch connects to an external monitor via HDMI.