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.