I don't cover lettuce seed, it needs light to germinate.

I plant lightly water and than cover the flat with a piece of plastic (can be any color or clear) to keep moisture in. I check the flats daily making sure they do not get dry. I generally get great germination unless the seeds are more than 3 years old.

Ditto on 3 years for lettuce seed, sometimes only 2 . Another thing I noticed is that poor germinating lettuce also grows poorly . I plant inside and out and as long as its not too deep it should come up easily light or not . I cover all my seeds the same and use the same depth setting for all the spring seeds on my 2 seeders and plant 1000's of row feet each year