just don't/doesn't get it, you/he/she

(one) just doesn't get it

One does not understand at all, often despite one's best efforts or intentions. The phrase typically refers to something that affects the speaker emotionally. A: "Your history teacher says you've been skipping class." B: "She just doesn't get it. Learning about this topic has been filling me with anxiety." A: "It stinks that you're injured right now, but there's always next year." B: "You just don't get it—so many scouts are coming out for the championship that it's a huge missed opportunity!" I just don't get it, Beth—why would you do something so reckless?
See also: get, just
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

just don't/doesn't get it, you/he/she

You don’t (he doesn’t) catch on, or really understand what’s going on. This phrase gained enormous currency in the 1980s when presidential nominee Ronald Reagan used it in a televised political debate with his opponent. Thereafter it was used often, particularly where the speaker is exasperated with someone’s failure to understand something. To get it has been a colloquialism for “to understand” since the late nineteenth century.
See also: get, he, just, she
The Dictionary of Clichés by Christine Ammer Copyright © 2013 by Christine Ammer
See also: