Most of the problems with lettuces not hearting properly and then 'bolting' to seed
are due to being too warm and/or drying out temporarily. They like the soil cool and protection from hot afternoon sun may be a good idea. Mulching will help retain soil moisture and temperature but keep the mulch away from the base of the plant
a bit. Lettuce like to be fed regularly with complete liquid fert, best twice as often at half the rate. If they are set back they don't recover well, they just stay loosely formed and can bolt.