I am having this problem also. One of my Cream Legbars has not laid a real egg in about 2 weeks but every couple of days I find a water balloon egg that I think may be hers. They are all eating layer feed, free ranging, have grit provided etc. She is the only one struggling.
It's likely stress related, assuming they aren't pullets at POL nor old hens. Something is causing alot of stress to the hen/s so check for predators, change in flock/pecking order, or space. Otherwise it could be related to the hen being overweight