Answer: Coreopsis prefers well drained soil, meaning soil that is not soggy, especially during the winter. When grown in a poorly drained location the plant simply rots over the winter and then fails to come back.
If it was planted in a low spot, you might want to try a different location. If drainage is poor due to heavy soil such as a clay, then you would need to work in lots of organic matter such as compost, rotted leaves, or aged stable manure and bedding, along with a small amount of ordinary coarse sand or grit. This will incorporate more air spaces in the soil. You could also use a raised bed, or simply raise the soil in that area up a few inches over the surrounding soil, in addition to improving it.
Another possibility, if you grew the lanceleafed coreopsis that blooms and blooms all summer much the way an annual might, is that it exhausted itself and died out for that reason. If this is the case, there isn't much you can do.
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