Front facing vents blows out everywhere else?

My air and my heat work perfectly fine. Whenever I go to put it on were a blows out at the feet or the defroster or what not it blows out normally but whenever I try to put it on the front facing vents that would blow on your face and stuff it starts making a knocking noise and it blows out everywhere else out of all the other vents except for the ones that face you. Is the thing inside of my vents stuck or is it a possibility it’s broken? But my main question is how easy is it to get to it to see if it’s stuck or broken? I was told I might have to take my dash off to check.