Yes, I agree, left hand position. Also whole body position has an impact. The left hand should be a relaxed 'C' shape. Straight wrist. Use the shoulder muscles to pull the arm back, which in turn pulls the hand and fingers to make good contact with the string. Only use the pressure you need to play a clean note - most people use too much.
Also well set up bass.
As mentioned by others Alexander Technique can help and anything that makes you more aware of your body and how you are using it.