Cherie's first post is SPOT ON for why this happens, and how to change it.
You may not have harsh hands. Could be an old habit. AND, sometimes when holding reins neither looping loose, not tight enougth to follow the mouth, you get into that place where they are tightingin, loosening, tightining loosening. Horse gets snapped in the mouth with each stride . Hard for horse to make heads or tails out of rein contact that inconsistent MORE but consistent and meaningful contact, ala Cherie's explanation , is the way to getting them to learn to drop head. THEN you must learn how to always follow mouth well OR stay totally off the bit. No imbetween.