Wilting lettuce is likely caused by dry soil. As with any plant, lack of water causes the plant to suffer. Lettuce requires a minimum of 1 inch of water a week, and it likes for that to be evenly dispersed. Avoid watering past 10 a.m. to reduce the instances of fungal diseases. It is also helpful to avoid wetting the leaves, but not always possible.
Lettuce is a natural slow grower in the vegetable garden. It takes its time to first appear, and may take weeks to show any significant growth. Many factors affect the growth rate, from climate to weather, from soil to temperature. Slow growth should not cause concern as long as the seedlings appear strong and healthy.
Another form of slow growth is poor germination. Lettuce seeds need a spongy moist growing medium to germinate, and if the seed bed dries out, the seeds cannot germinate properly. Wrapping seeds in a moist towel and placing them in the refrigerator during the day and on the counter at night can hasten germination of the seeds. Old seeds may also not germinate well. Start each growing season with newly purchased seeds.
Many animals and insects like young lettuce as much as you do. Among these are rabbits, deer and slugs. Some types of fencing may keep out deer and rabbits. Slugs can be either eliminated by manually picking them off the plants, or by offering a shallow dish of stale beer for them to drown in. Slugs are less of a problem as the plants mature, preferring instead the delicate young seedlings. Seedling can be wiped out by slugs if left unchecked.