elementary OS 5.0 Juno. Three Options How Suspend Mode Works. 1) Suspend working correctly - The screen is black, disks sleep, the power supply sleeps, the button of a power supply of the system unit blinks, 2) The screen is black, the power supply works, the button of a power supply of the system unit blinks, 3) The screen is black, but the computer does not go to sleep
If elementary OS is the only OS installed on your PC, it's hard to tell, whether it's elementary OS' fault or Ubuntu's - as elementary OS Juno is Ubuntu-18.04-based.
I'm dual-booting elementary OS with Windows, and my PC suspends normally in both. My PC's power LED is off during sleep/suspend, but as far as I know, on other PCs it may as well blink, because this behavior is motherboard-specific.
Anyway, as this random dysfunction might be hardware-specific (old PC, failing HDD) or software-specific (some process or app messing with power settings and preventing normal suspend), you can upload your logs to a hosting and update your question with a download link, so more technically proficient users than myself could have a look at them.