Answer: There might be something in the soil that affects the tomato plants or it could be that the area remains too wet (a patch of clay, perhaps?) I'd be curious enough to dig down about 2' to see what's there. It's always a good idea to rotate your vegetables to different spots in the garden because disease pathogens and insect populations specific to that plant family may remain in the soil, just waiting for that particular crop to be planted again. Eggplant and peppers are members of the same family as tomatoes, so don't plant those in the same areas, either. If you wait 3 years before planting your tomatoes in the same garden spots, they should be safe from problems lurking in the soil.
Best wishes with your garden!
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