Short answer
Start by estimating how much of the plant is affected, because transplant shock is common and often not the end of it. Extension guidance draws the practical line at more than half the plant dying back; below that the advice is to keep watering correctly and give it time rather than to dig it up. The most commonly missed cause is the opposite of what it looks like: the original root ball can be bone dry inside a hole that has clearly been watered, because a dried root ball repels water and a new plant can only drink from that small volume until its roots grow out. A new planting also needs water far more often than the rest of the bed, roughly daily for the first two weeks.
Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.
Montana State Extension states plainly that transplant shock is common in newly installed plants and that delayed growth is common after recent planting, with compromised roots as the primary cause. Leaf scorch, wilting, reduced leaf size and yellowing are the listed symptoms, which is most of what makes a new planting look like it is dying.
What to check first
Estimate how much of the plant is affected before doing anything else. Georgia's extension guidance draws the practical line at more than fifty percent dieback, and below that the recommendation is to keep watering correctly and give it time rather than to intervene.
Clemson's extension guidance is explicit that dry burlap repels water, making it difficult to rewet the root ball, and that initially the root ball needs to be watered directly because roots have not yet spread into the surrounding soil. So the ground can be damp while the only part the plant can drink from is bone dry.
What to check first
Dig a small hole a few inches from the stem and feel the original root ball itself, not the soil around it. It is usually a different colour and texture from your own ground, and finding it dry while the surrounding soil is moist is the answer.
A new planting needs water far more often than the rest of the bed, and two extension services give near-identical schedules for it: water daily for the first one to two weeks, then every two to three days from weeks three to twelve. Weekly watering, which is generous for an established plant, is well short of that.
What to check first
Compare how often this plant is watered against those first-weeks numbers rather than against the rest of your garden. If it is on the same rota as beds that have been in for years, that gap is the likeliest answer.
Illinois Extension puts it in one line: planting too high results in plants drying out and too low invites crown rots. The same guidance says mulch should not be applied right up to the crown of the plant, for the same reason, so a correctly planted crown can still be buried afterwards by the mulch.
What to check first
Pull the mulch back and find where the stem widens into the roots. That flare should be visible and sitting at or just above the surrounding soil, with bare ground in a ring around the stem rather than mulch banked against it.
Georgia's extension guidance explains that plants put in during the cooler months can establish while dormant, and that spring-planted stock is more stressed from summer heat because their roots are still underdeveloped during the first year. A plant put in during the hottest weeks is asked to replace what its leaves lose using roots it has not grown yet.
What to check first
Think about when it went in rather than what you did to it. If that was during your own area's two or three hottest months, heat on an undeveloped root system is a live explanation even when everything at planting was done correctly.
Worth ruling out for next time rather than as an explanation for what just happened, and the sources are clear about why. Florida's extension guidance describes a root mass so dense that it prevents roots from penetrating into the landscape soil after planting, and Maryland describes a root growing in a circular or spiral pattern around the trunk, gradually strangling it.
What to check first
Dig gently at the edge of where the pot went in and see whether the roots still hold the container's outline. If they do, this is present, though the timing below argues it is probably not what is happening this week.
Still not sure
This is exactly what your county Extension office answers, for free. They are the local horticulture experts for your area, they know what is going around this season where you live, and they will look at a photo.
Find your local Extension officePollinator Patch gives you watering and seasonal care for the plants you already own, written for your region rather than for gardening in general.