Why You Can't Putt
It's not enough to have your putterface square at impact. To putt consistently well, it must be square through impact, an inch or two on either side of the ball. However, many golfers instinctively turn their bodies toward the target as they strike their putts, sometimes as much as 20 or 30 degrees off the target line. This matters because the shoulders sit on top of the hips, and when the hips turn, the shoulders turn -- and then the putterface turns: Your timing has to be perfect to start putts on line with this action.
Forearm rotation also plays a role. You want to rotate (or release) your forearms through impact in most golf swings, but never in putting. Forearm rotation helps produce the power needed for most shots, and to get that power golfers must live with the inaccuracy in shot direction caused by clubface rotation through impact. But you don't need forearm power when putting; you do need accuracy, which is destroyed by putterface rotation.
First diagnose whether or not you have a problem with lower-body rotation. Stand in a doorway with your left (targetside) hip pressed lightly against the door jamb and your shoulders, arms, and putter able to swing freely on the other side. Take some strokes: If you feel your hips trying to move, or if these swings feel different than your normal strokes, you are moving your lower body on the greens. To cure this, close your eyes and focus on the feel of an arms-hands-and-putter stroke. Feel no hip rotation. Keeping your hip up against the door jamb, work on this for a minute or two each day for several weeks and you'll eliminate the unwanted motion.
Forearm rotation is harder to feel and eliminate. For most golfers, a little arm rotation feels more natural than no rotation at all because your arms have been rotating in almost every other shot and you've never thought, or realized, that they did this. Also, in changing from what you thought was not turning to really not rotating, it feels as if you are turning your forearms under. But feelings are relative, so don't be fooled by them.
To eliminate forearm rotation, attach a perpendicular pointer about six inches long to the face of your putter (use double-sided tape). Without a ball, take practice strokes, making sure to hold your finish. Look down at the finish to check if the pointer is aiming down your starting line. If not, correct the position by squaring your forearms. Repeat with your eyes closed until you learn the feel of following through squarely down the line to a perfectly square finish.