be (a little) slow on the uptake
To struggle to understand something. This phrase is often used as a euphemism for stupidity. Joe's a nice kid, but tutoring him is a challenge because he's a little slow on the uptake, you know? That group is slow on the uptake, so I don't know if they'll ever understand how to use this fancy new printer.
be ˌquick/ˌslow on the ˈuptake
(informal) understand things quickly/understand even simple things with difficulty: He’s a very good worker but he’s a bit slow on the uptake sometimes. You have to explain everything twice.