Lettuce is easy to sprout and grow. I can only guess that perhaps you have an older batch of seeds? or the seeds got moldy or damp or something like that. Often, in a greenhouse, the sunny day temps can reach very high and then at night the temps will go very low. So my only guesses would be the temps or the batch of seeds.
~tom

Thank you Tom,
I hadn't thought about it being the seed. I was still using last years package. I will try again with a new package of lettuce seeds and see what happens.

thank you,
cristi

They are also cool weather plants and like temps in the 50s or 60s for germination. If you wrap the flats to keep moisture in or use humidity domes, try poking or drilling holes in the covers to lower temps inside. You can try germinating them closer to the floor and to the greenhouse walls, where it may be cooler.