Need to figure out why they're struggling before you do anything. When a plant is stressed -- aka struggling -- losing resources by being cut back is often a death knell.

Jean001a is right, I think. I also don't think fertilizing a struggling plant would be wise either, at least not until you have a better idea of what's going on. When a plant is struggling, doing nothing is difficult, but it may be the right thing to do.
What do the leaves look like? Can you take one to a local nursery and ask for a diagnosis? Not always a reliable opinion, but at least it would be a start. The other possibility is to use the resources of the internet, look up something like "camellia diseases" and see if anything you pull up looks like what you've got.

Kay

It to hard to say what is wrong, if you post a photo of the plant and include the soil in the photo. Is water getting to the root zone? If the ground is hard it may not get water, but too much water means root rot. It could be a disease also. Don't use a chemical fertilizer now, try soil improvement to make the soil better first. Chemical fertilizer can burn a stressed plant causing more stress. If you use something make it something light such a miracle grow for acid lovers in a very weak concentration. Do a soil test if you can to see if your soil is acid enough. If you add some Aluminum sulfate don't add too much, mix it with water.

Thank you. The three new camellias are planted in the same area, so it's hard to know why one is having trouble and the others are not. But all the older ones had a hard adjustment period too, and they're now flourishing. I think I'd better protect them with screening and take it easy with the fertilizer.

It is not normal for Camellias newly planted to "struggle". Maybe this is a good time to go over your "planting a new plant from a container" routine. Al