It could be a shell gland defect since she is over a year old. Some hens just seem to have problems. Observe your flock to see if she is under any added stress, see that she is eating/drinking (if being kept from food add feed stations). Offer oyster shell free choice.americauna hen that’s a little over a year old & is very inconsistent at laying. Her eggs are very small & this past weekend, she laid an egg without the shell.
It's never a bad idea to take some poop samples to your vet to rule out worms, but generally malformed eggs and inconsistent laying is a reproductive problem.
Here's more information about egg issues:
http://www.yellowbirchhobbyfarm.com/weird-eggs-101-the-oddities-explained/
http://www.backyardchickens.com/a/common-egg-quality-problems