In vim, leader key won't work

I've searched on internet, and there are people asking a same question but without satisfying answer. So, I'm trying my luck here.

I'm using vim in terminal. And the leader key won't work. I've tried to remap it to ",", only to find it not working as well.

Each time I type the leader key, it beeps as if it was an empty map, however when I "echo mapleader", it shows my customized map leader ",". So what's wrong? Why doesn't map leader working? I mean, I used to work on linux, and I know it should work on bash.