I'm having a similar issue with TWO of my several breeds. My Speckled Sussex's and Golden Lakenvelders, housed next to one another, separated only by chain link fence, are not laying. The Cuckoo Marans' on one side, and the RIR's on the other side of them, are laying pretty well. About 50% on the CM's, and 75% on the RIR's, weekly average.
I've not gotten an egg yet, out of the Golden Lakenvelders, since getting them in January. The Speckled Sussex's have given me about 5 total eggs, in the same time frame. I have 3 GL hens and a rooster, and 7 SS hens with 3 roosters.
The local "expert" at the Co-Op, said full spectrum light, on a timer, to come on around 2am, adding Karo syrup to their water, and adding green leafy veggies to their feed, should help. It got me one egg, two days into this attempt, but nothing since. Fluke....probably!
A friend, who runs a large commercial egg laying operation, this morning, suggested not just the Karo, but adding 1/4 cup of ACV, to every 5 gallons of their drinking water. He believes ACV to be a miracle supplement for chickens, so, I added some, this morning, and we'll see where it goes. But, he has 20,000 chickens, and gets 10,000+ eggs per day. I presume he knows what he's talking about.
I hope this helps, as it means it helped me too. But, in searching for an answer to my problem, I found this question, and thought that I'd pass along what I was told.