I planted some red romaine lettuce in our planter box about a week and a half ago. It seems to really be struggling. I water it about once a day and it gets 6-7 hours of sunlight a day. Any suggestions would be appreciated!

Lettuce is a cool weather vegetable. Mine just quit with temps in the upper 90s. Some type of root rot is possible, but lettuce is very prone. Hot and wet conditions do encourage fungal rots.

Do they recover at all overnight; are they better in early morning? If it's just not enough root system, maybe you can shade them.