I've always found it to be a software problem. There's a lot of software that inhibits the computer from sleeping. I use Synergy to use a single keyboard/mouse

I've always found it to be a software problem. There's a lot of software that inhibits the computer from sleeping. I use Synergy to use a single keyboard/mouse across multiple computers. It also seems to completely inhibit sleep in whatever computer is running a client. Then there's shit like Discord or Firefox or Steam. Any time you run a full screen app or play music, the OS seems to think "oh hey you're playing a game" and then inhibits sleep.