Sorry for the multiple posts… thinking in strings
Lettuce is a cool weather crop… let it go if need be and try again end of summer. Farmers Almanac online is the best to tell you when to plant what in your zone.
Last year my peppers and melons stalled for what seemed like forever until it got really hot like over 80 then they just kicked into high gear. Some summer veggies are like that so don’t get too discouraged.
Whatever you do… don’t let anyone talk you into manure. I put tons of it in my patch and planters before I knew better and it raised my ph and threw all my mineral absorption out of whack. I am reaping the consequences this year as opposed to the last year I added it. Also, no more hydro nutes out there. Try simple Dr. Earths slow release dry and leave it be. Something mild and slow release. For a quick nitrogen fix try liquid fish or kelp.