First, Greetings @TinleyFarms from the front range in Colorado ! Sorry to hear about your egg woes... I can identify! I have almost 30 hens (all are over 18 mos old), and am getting 1-2 eggs a day if I'm lucky. At their peak, I was getting 18+ a day. Some days now, I get none at all, and other days I have egg eaters that wait to lay until right after I've checked... I go back out later and find a wet spot and maybe a little left over egg white... the yolk and sometimes the entire shell is devoured. All it takes is for one to start the job and the rest all join in and the egg is gone in minutes or faster. Talk about a bunch of velociraptor cannibals!! If I could catch the initial egg eater, she (or he) would be soup stock in under 24 hours!
There are a couple of things that may be contributing. The youngest may have not started laying yet, and may now wait till January as the days get longer again. We just passed the winter solstice and entered the winter season, so now the days will steadily get longer once more. Typically, a lot of chickens shut down egg laying as the days get shorter. A way to see if this is in fact the issue would be to install lighting in the coop and add extra hours of "daylight" so they have light for 14 or so hours a day. Many folks do this to keep the birds laying year round. I don't do this as I like them to have a couple of months rest, as nature intended, before they go back into max production. If you install lighting, it may take a week or so for it to register and egg laying to start back up again.
Some have mentioned molt, but I believe you stated that none have ragged feathers. A molt doesn't necessarily mean they go bald... It's not like trees losing their leaves... Some chickens can molt out completely over time but they do it so slowly and over a longer time frame, that you wouldn't even notice. Have you noticed more feathers on the ground in their area at all? Oh, and this doesn't always happen either as some chickens eat the feathers... they are protein after all. Most of mine have just finished molt and are almost completely re-feathered now. In the past several days I have started to see an increase in egg production, but still nowhere near what they were doing at peak.
Scratch grains should be more of a treat for the birds and should NOT be fed free choice. I have tried all sorts of protein levels 10%-30%+ and have found that anything over 14-16% is pretty much wasted. Chickens can't really put the additional protein to use... they just poop it out. If you're feeding layer feed, there's already added calcium in the feed specifically because it's designed for laying hens. As such, they may not need/use the oyster shell. Another interesting thing is the amount of protein in the feed determines how much the chickens will eat. As a result, when feeding a high protein food, they don't eat as much as their systems tell them they've met their daily requirement. The result is, they don't get all the nutrition they actually need and will lose body mass and tone. This can also affect egg production as the body has fewer fat stores and such to draw on. I am now buying 16% layer feed and mixing other grains in with it to draw down overall protein content. I normally ferment their feed but with sub freezing temps, I can't do that right now. I am right now trying to get protein levels to between 12-14%.
My recent improvement I attribute to the lower protein level in their food as well as the molt being almost completed. I hope it gets better for you soon. Good luck, and welcome again!