At 21 weeks old I wouldn't give calcium if you've only gotten the one soft shelled egg. It's normal and expected to have some glitches with onset of laying.
It doesn't work that way, they lay where they think they should lay. If that's the same nest, then that's where they lay. Some birds on...
If she's eating and drinking fine, and otherwise active and acting as she normally would, I wouldn't worry about it. Not unusual for young layers to be a little glitchy, but would be unusual for a young bird to suddenly develop reproductive issues, especially without showing signs of discomfort.
I'm dealing with it right now (with older hens, not new layers) - one hen came back into lay about maybe 6 weeks ago, laid maybe 12 eggs, and then stopped completely. No one else has come back into lay until 2 days ago. Frustrating, since I've long since run out of saved up eggs!