Why can't I play a Bar chord on a guitar?

I can't really play ANY bar chord. It seems like the sound is muffled or what. It sounds like I'm not pressing the chord hard enough. Okay I'm a newbie been playing guitar for 3 weeks. I've had calluses. And I can already play a whole song and switch my fingers fast but i can't still play a bar chord. I've watched many videos on how to play a bar chord but still can't do it right. How long would it take for me to play that chord? any suggestions on how to?