you just don't get it
see under get it, def. 2.
Is It “Just Deserts” Or “Just Desserts”?Dictionary.com once featured the word comeuppance as a Word of the Day. Comeuppance is defined as “deserved reward or just deserts, usually unpleasant.” More than a few devoted users wrote in to inform us that there was a typo in the definition: just deserts should be just desserts. Were our users correct? Was an s left out of this expression by mistake? Is the phrase just …
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.