Drop your jaw a little and breathe blow/draw with your diaghram. Be sure your harmonica is far enough into your mouth, and your lips should feel soft and relaxed.
Hi Chandan....what make/model harp are you playing? A low quality model could be harder to play than a decent one. However, technique and the practicing of your technique is important to get good tone throughout the instrument.
As Gene has said, try breathing thru' your diaphragm - and try not to force your breathing.
With draw notes, getting the reeds to vibrate correctly, you will find that if you apply the term "play gently" a rounder more full note is produced. Do not get frustrated and draw in lungs full of air thinking that this will get the damn thing to play. That can break your reeds and you end up chucking it in a draw and saying bloody thing I can't play that. Calm down and play gently at this early stage there is no need to draw the roof down.