I have been growing head lettuce and baby leaf lettuce for about a year. All of a sudden I can't get my had lettuce to germinate. I sow in a light potting mix in 72 cell. The soil is nice and moist, but not wet. I either sow directly on the soil surface or sew and then cover with a light layer of vermiculite. I then cover the container with plastic wrap to contain the moisture, place on a grow table under light and ... nothing happens. My seed is good because I have sown it directly in the grown for baby lettuce and had no trouble with germination. I have also successfully germinated these varieties previously. Any suggestions would be appreciated.