is it only your front squat that is problematic or is it ALL your squats?
i had trouble with the roll under and it turned out i needed to learn to hold my arch 'sitting into' my hip flexors (strange feeling). for me the issue was one of motor control / proper neurological recruitment.
the issue might be different for you... but i've found a few people who thought they had tight hamstrings... and it turned out that the issue was neurological recruitment.
check out ripptoe's article 'back position for power', too. might be that you are having trouble knowing what it feels like to hold / maintain the lumbar curve through the ROM??