Is your Mac messed up and not activating the corners like you have chosen. You have two options. Using Activity Monitor, or if you’re more adventurous, you can use Terminal.

Let’s start with the easy way. If you go to Applications -> Utilities -> Activity Monitor. Organize the entire list by clicking on ‘Process Name’. Locate ‘DashboardClient’ and Highlight each one, and click on ‘Quit Process’.

For the more adventurous, If you go to Applications -> Utilities -> Terminal. Type in sudo killall DashboardClient. This will kill every instance of DashboardClient that is currently loaded.

Now, try your ‘Hot Corners’ to see if they work. If they don’t want to, you may need to logout and log back in.

Just a quick tip for those who might be experiencing the same issue.