Answer: Since all of your other plants are doing well it sounds like there is something wrong with that particular plant. I suspect it has failed to root into the surrounding soil. This can happen for a number of reasons, the most common being that the plant was severely pot bound prior to planting and the roots have continued to grow in that tight mat instead of spreading out. You might want to dig the plant up and see if this is the case. If it was potbound, you may still be able to tease some roots loose and replant it with success. If you do this, treat it the way you would a new transplant afterward.
One other possibility I can think of is that the plant was somehow severely stressed before you bought it and for that reason is just not going to thrive no matter what you do. If this is the case, your best bet would be to replace it.
Alternatively, there might be something wrong with the soil in that particular location (perhaps something like lime or fertilizer or gasoline was spilled there at one time) or perhaps it is growing in a particularly low spot so drainage is poor? Occasionally there is some other "mystery" cause such as a ball smacked it or a cat urinated on it. Such are the puzzles of gardening!
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