Its probably a root, and/or water and/or soil issue. Often times we see plants in a group or row do this for two reasons: 1) If the plants have been recently planted, this plant has not developed a proper root system for the needs of its above-ground growth. This could occur due to extreme differences in the ground soil and the soil/amendments put in the hole around the plant. If the soil around the hole is very hard and does not drain well, it can act like a bowl and hold too much water at the root zone. 2) For newer or older plants, it could be due to excess or not enough water for the soil composition in that area. You would think that soil in the same area is exactly the same throughout, but it can be different enough in the area of this plant’s root zone to creat a problem; and sometimes we don’t realize the water is either draining toward or maybe away from a particular plant. It would be important to know if the plant wilts during the warmest part of the day, and then recovers in the cooler evening and morning hours. This would indicate a root rot issue that could be encouraged by too much water in the soil (for the soil composition in this particular area). We recommend checking for irrigation leaks in the immediate area, proper operation of any irrigation in this area, and appropriate watering schedules for these plants. If you can correct any issues, the plant should recover but it may take some time. If the plant continues to decline, we recommend pulling it out of the ground and examining the root growth for white healthy roots (vs. brown, very wet roots), and soil moisture. The soil should be moist, not wet and not extremely dry. If you find the roots have not grown due to extreme soil hardpack, correct any issues with hole preparation, mix ground soil with well draining soil amendments to put back around the root ball of this plant and water properly. If the roots are rotten, correct any issues and replace with a new plant, planting as described. Keep in mind that if this is being caused by a fungal pathogen in the soil, the disease will be encouraged by excess moisture and putting most any plant back in this same spot without proper soil and water may cause the same problem again. You can allow the area to dry completely (under as much heat as possible) to try and reduce incidence of diseases that occur in soil.