Having got a new greenhouse last year, for first time I tried to grow my own lettuce, I was very excited about it. They didn’t gow very well! I was very disappointed. I think (a) mayybe I transplanted the seedlings too early, they were very tiny, or (b) maybe I overwatered them?
Sow the seeds where they are to grow and thin them out carefully, or sow 3 or 4 seeds to a small pot and thin ot the strongest and plant it out carefully.