Bumpy crust on eggs

Hi,

I have a young flock (7months). They all seem healthy, we feed them a mix of layer pellets, scratch and veggie/fruit clippings. They have access to the outside throughout the day. They have plenty of water and we have a light to regulate their 14 hour light cycle. They are each laying an egg a day, most days.

Here is my problem: sometimes one of the eggs has a bumpy crust on some spots. It is firmly attached to the shell, and the egg inside is perfect. It looks like it is part of the shell and formed attached to it.

I am wondering if I need to do something different in their diet??

Thanks for your help!