Mac mini M4 won't sleep

I have just purchased a new Mini Mac M4.  It runs Sequoia 15.1.

All seems great except I cannot get it to sleep reliably without shutting everything down. Is this a known problem?  The screen goes off then after a few seconds comes back on.  I have trawled various forums looking for solutions and have not found anything that works.

I’ve tried…

-              Checking activity monitor to see for apps running that would prevent it sleeping (there are none)

-              Shutting down all the open apps and relaunching Finder

-              Logging out, then requesting sleep (no there are no other logged on users)

-              Messing about with screen settings.

The account is set up with no screen saver, to shut down the display after 5 mins and request login a minute after shutting down the display.  The apps I used most are Safari, MS word and MS Excel.

Occasionally if I just leave the mac for 5 minutes  it will turn off the screen but definitely not always and I have no idea if that means it is asleep or has just switched off the display drivers.

One thing that seems to work (but I have not tried it enough to know if it is reliable) is logging off then restarting.  If I do that then attempt to sleep it, it will sleep but as soon as I log in, I am back to square one.  

Far from ideal, I would just like to sleep the computer on demand without having to shut down everything I am working on.

I have a pretty old Mac book pro (10 years plus old).  Sleep works perfectly on that.

Can anyone confirm if this is a known problem or provide a reliable solution?