Mine are mostly volunteers and are delicious - super lettuce weather. Bitter usually means to much heat. As to the impact of too much fertilizer not so sure. Look at my nearby post "Red Gold"

Nice looking garden. :) lol My lettuce looks very sad compared to that. Hmmm... Don't know what to do.

Have they had a lot of water? What you describe sounds like stress. Stress for lettuce often means hot and dry.

ltilton - My lettuce bed has been wet enough this week that mushrooms are growing in it, but I has been pretty hot theses last couple of days. I'm thinking I'll just pull them and put in some okra... Well eat more of that anyway. :)

They don't look like typical butter lettuce. If you want lettuce I would try another variety. "Buttercrunch" is usually great until the weather gets hot. I will be trialing a variety "Carmona" which is a red butter lettuce for warmer weather but the seeds aren't up just yet. Or just plant okra. I personally don't have warm enough weather here to pull that off most years.

we've had plenty of too-hot-for-lettuce days, in between the nice cool with a bit of rain days.

I covered my lettuce when it was too hot, the black seeded simpson still tastes good.

check your prior weather, and see how may days it's been over 80 degree for you - if you didn't shade, that might be an issue.

I'm not seeing any pictures. What are you guys looking at?

I don't see any photos either. Bad tastes in lettuce usually come from too much heat or too much dry or too old. The outer leaves are the oldest and most highly flavored. Butter head is a heading lettuce and as such it is best used by harvesting the whole head. Pick off the outer leaves and try the inner ones. It may taste much much better. I don't always eat the outer leaves on my romaine for the same reason.

I heard that if you soak the lettuce in cold water for about an hour after harvesting it will not be as bitter, also the longer it's refrigerated the more it will lose it's bitter taste.