Answer: It is a bit difficult to make a diagnosis without seeing the plants but an all over yellow color could indicate a lack of nitrogen, perhaps you omitted a top dressing of nitrogen at planting or subsequently? Lettuce are relatively heavy feeders that need a rich soil with ample nitrogen along with constant soil moisture, the soil should be damp rather than saturated/sopping wet. Or, the pH might be too far from the ideal of 6.0 to 6.5. Or, there might be a drainage problem. (Sometimes soil in a raised bed can vary from spot to spot due to the different ingredients used.) Or, if these were transplanted, they may have sunburned if not sufficiently hardened off prior to planting in the garden. It might also be a disease such as mosaic. You might want to consult with your local county extension and see if they have any ideas based on seeing photos (overall and a closeup) and/or a freshly cut sample kept cool and in a closed plastic bag. I'm sorry I can't be more specific for you.
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