Check advanced power profile settings, probably you've got "Hibernate After" set to a certain amount of time. Sometimes this auto-switchover fails. Workaround by disabling this, set to Never (0).

After this happens, the laptop seems mostly to run normally, but some things are weird, like this insomnia thing you report. Then only hard shutdown (5 secs pwr button) works.

Also try reflashing BIOS because this resets things (if you do BIS reset, you have to redo changes you may have made).

Hi,

thanks for the answer. I've set the hibernate after to Never, but it didn't help. Then I restarted with power button for 5 seconds, and after the restart I was able to hibernate the computer. Today I tried it again, and again I was able to do it. So I will keep on testing, and will let you know whether the problem is gone or not.

Hi,

so I experienced the issue again. It seems that some time after reboot, it is working, however, after some time, I still can't hibernate the computer.

I didn't try your last advice about reflashing BIOS as I don't know how to do it, and since it is my work computer I am also bit afraid that it might break something.

Hi,

Not to worry, we will look into this. Kindly DM and let us know the current symptoms that you are facing. 

I am having exactely the same problem with my XPS 13 9380. Since it updated to Windows 20H2 last month it won't sleep or hibernate. Unfortunately I cannot return to the previous version of Windows because I deleted it (it horded 35GB). 

Very frustrating. I have to fully shut down the computer every time I am done using it. 

We have the same issue, it's infuriating.

Hibernating works only every other time, most often it immediately wakes up again after hitting hibernate. It is so frustrating.

Same here. 

XPS 13 9370. Windows Version 10.0.19042 Build 19042

Does not seem to be a common factor involved. First I thought it had to do with certain programs bein active, preventing the system to go into sleep, but that does not seem to be the case.

Having this same issue - is there any fix? Dell, are you aware of this issue and working to resolve it?

I found something that helped on my 3 month old XPS 8940. It was having the same problems that were caused by not shutting down correctly, even though I followed all the right shutdown procedures. Hybrid Sleep is the new default for shutdown. Shutting down completely now requires pressing the Shift key when selecting Shutdown. That cured the shutdown problem. Some other tips that might help are included in this--

How To Properly Shut Down A Windows 10 PC (lifehacker.com.au)

Having the same issue. It's frustrating since most Macs can be working for weeks on the same session, whereas I always have to save all my files and close windows when I have to move my XPS9370 to work, for example (unless I keep it running, which uses all the battery)