Thin, puckered shell from new layer

I've got 5 Red Production pullets about 19 weeks old and just starting to lay. The variety of egg shape, size and shell has been amazing. One gal has produced at least 2 eggs with a thin, puckered-looking shell. Could this be simply growing pains? She also has a hard time making it to the nest box, either dropping it right off the roost in the early hours or just leaving it in the middle of the run. She is lowest in the pecking order and gets bullied away from the nest box. We were told to have only one nest box for 5 birds but I wonder if that is enough.
They have constant access to layer pellets and oyster shell, an outdoor run and several hours free range in the fading veggie garden.
Thanks for any and all advice, both in the past and the future!