If you have closed the cookie banner once before, the banner will not show to you again (since it’s considered as you accepted the Cookie Policy), to re-enable the cookie banner again for all users you just need to change the Cookie ID field (in Theme Options > Settings > Cookie Consent) to any value not used before (for example Cookie-2) and then save the changes and the cookies banner will show again.
For more information about adding the Cookie Consent, you can check this article: