Everybody's hands are different, therefore, the grooves in each person's left hand index finger are in different spots. The problem occurs when one of the strings in one of your grooves. For me, I always had trouble playing the third string clean. There are a few things you can do, though. First off, try positioning your first finger higher or lower to see if you can avoid the grooves. Also, if you rotate your finger slightly to the left the grooves are not as pronounced and you should be able to play cleaner. My final suggestion is trying to make your bigger muscles do the work. (Try this sitting down)Instead of squeezing really hard between your index and thumb fingers (which is a weak muscles) try pulling your left arm back against the fretboard. You don't even need the thumb on the guitar for this exercise. If you were to just let go of the guitar and drop your arm, it would not go straight down to the floor, it would also go back towards you. Use gravity and the big muscles in your arm. You could also try pushing your body forward against the body of the guitar while your arm is pulling back to create an even stronger force.