Most often shell-less egg are a hiccup of the reproductive system getting itself in order. Usually very young layers just coming on line. Or ladies that are entering or exiting their lay cycle from molting type thing.
Where are you finding them? How old are your gals? What do you feed including treats and supplements?
It is more common when chickens first start laying. I assume that you have a supply of crushed oyster shells or other calcium source for them? I sometimes mix some crushed oyster shells with their feed.