Are you booting in UEFI legacy mode on purpose?
I think there might be a kernel option you need to specify on the GRUB_CMDLINE_LINUX line of /etc/default/grub for suspend to work correctly if you are booting in legacy mode. Let me see if if I can find it.
Edit:
I’m not as confident this will fix your issue as I initially was. I found the kernel option I mentioned, but it doesn’t appear to be a universal fix (some people try it and it doesn’t help).
If you want to give it a shot, it’s easy and it couldn’t hurt. You can always delete it out if it doesn’t work.
sudo micro /etc/default/grub
A few lines down (line 7 on mine) is GRUB_CMDLINE_LINUX. Add mem_sleep_default=deep to whatever you already have there.
sudo update-grub
Reboot.