This plant does like full sun - it also likes moist but well drained soil which is quite gritty or gravelly. If you only planted them a few weeks ago, the likeliest problem will be lack of water at the root - although this plant is quite drought resistant, that's only once its fully established, so initially, it needs plenty of water. Irrigate or thoroughly saturate them to see if they recover, and water copiously on a regular basis. If you're not sure that's the problem, dig one up and inspect the rootball - if its dry, then you know its drought.

Thank you, Bamboo, for the speedy reply! I'm trying your idea, irrigating thoroughly and watering copiously.
I did not dig one of the plants up, but I'll see how it goes and let you know!

Yellow Thumb a.k.a. Bill