You're getting inconsistent contact off the face of your putter you leave some putts short and roll others well past the hole.
Make sure you're set up to use the loft built into your putter, with your weight evenly distributed and the handle pointing at or left of your navel. When you press the shaft forward or set too much weight over your front foot, you take loft off the face, creating a bouncing effect instead of true forward roll.
Also, take the time to discover the location of your putter's sweet spot. (Most people assume it's directly under the alignment aid, which is not always the case.) Hold your putter out in front of you and bounce a ball a few times off the face. The point at which contact feels the purest is your sweet spot. Mark it in pen, and align it directly behind the ball on every putt. That will take the inconsistency out of your contact.