Some backgound processes may caused that, as can oler USB devices. Do you have any anti-virus or so-called "cleaning/tune-up" apps installed? This Apple article may help:
If your Mac doesn't sleep or wake when expected - Apple Support
If you have a lot of USB devices attached (other than a mouse and keyboard), disconnect them, restart the computer and test.
If you are not sure what is installed (i.e., someone else did the iniotial setup for you or another had access to your admin acct) you can post a snapshot of your configuration that may help us better diagnose the problem. Fortunately there is a safe and secure way to do that.
A respected and long-serving member of these communities has created a simple utility that will take a "snapshot" of your configuration without revealing any sensitive information about you and your computer. It is Etrecheck, and is available here:
Run it, select Report" from the left-hand pane (scroll down to fond) and, when its report displays in the main pane, click the "Share Report" icon and then "Copy Report" from the resulting dropdown. Paste the entire report into a response to your own thread here. It will allow us to quickly identify or eliminate software as the problem.