Maybe it is bolting (going to flower/seed)? Seems lettuce may do better in cooler spring weather, and tends to bolt in warmer weather:
I am not any sort of expert on this but, you may want to pick individual leafs as they grow. Smaller leaves may be sweeter and then turn a little bitter as they get big. So, with any luck, you may be able to pick leaves all summer. I would also keep them watered.
Its too hot for lettuce now. Lettuce is a cool weather crop and its bolting which usually makes it taste bitter.
We have about ten lettuce plants we grew from scraps. They're doing well but they do not look like the ones from the supermarket. Instead of bushy bunches, ours are very tall with leaves growing apart. Still delicious but I want to know if I'm doing something wrong. I cut one to the base to see if it regrows...we'll see. In the photo here, the plants are about a foot tall.
TIA for any recommendations/suggestions.