Which one doesn't look like the other?
I'm not sure what happened here, to make one planter of lettuce robust and the others anemic. Maybe the anemic ones were planted in potting soil from last year, while the robust one received fresher potting soil. Or it could be the other way around. I suspect it is the former, as most of my seedlings also looked anemic, despite treatment with fish emulsion. Does potting soil go bad over the winter?
Last year's potting soil was the standard Miracle Gro stuff from Home Depot, while this year I purchased Miracle Gro Organic Choice, from Costco. The reviews of the latter are mixed, to say the least, but I still think it is the better of the two this spring, if only because it is fresher.
I raised this lettuce to give away, but I'm too embarrassed to gift any but the robust planter. It will go to my neighbor across the street. Maybe I can revive the other planters by side dressing with the newer potting soil.