They needed more light..that's all I see.

Perhaps the growth was also during too warm temperatures..lettuce is a cold season crop.

They're pretty stemmy. I think it's too hot for them.

Yes, it looks as if they have been too warm and possibly too dark. Are they indoors? They appear to be bolting already. I'd cut my losses and eat them now.

What is that potting mix made of?

These are actually outdoors. I have them set up out on the lanai. Temperatures here have been ranging between upper 70's to lower-mid 80's. Attached is a pic of what it looks like early in the morning. Much more sun will hit it during the mid day. If that's still too warm for lettuce to grow, can anything be done to compensate for the heat?

I've used Miraclegro seed starting mix in the seedling tray, and Organic Choice Potting Mix for this larger pot.

They look ok to me, maybe a little crowded if you want full heads. Keep them well watered and fed and give them lots of sun.