Actually I don't like to add calcium until they are actually laying. Some do, but I don't want to overload their kidneys.
I would guess that their point of lay time arrived with the cold weather, delaying them until spring. Marans aren't a breed known to really pump out the eggs like a Leghorn would, so it's not surprising to me that they are running a little late. Don't feel too bad; some breeds take longer, and now and then hens don't lay til nearly a year, or even over a year. I imagine as soon as it starts warming a bit, you'll see those combs brighten up and they'll start squatting for you.
You could certainly try the protein and even a little extended daylight if you want, though I'm not sure it would be worth it to go out and buy a light socket and timer this late, as the days are lengthening a fair amount already. If your layer is 14%, IMO, that's a bit skimpy anyway.
My question was premature. I found three eggs in a nest box this morning so somebody was already laying. I'll check those boxes more closely now. Not as dark as I had hoped but not to bad. I'm going to collect the first week and put them in the bator. Then with bators full, ( gotta finish my200 egg DIY bator). I may try letting the broody silkies do the job. I'll grab babies when they hatch because first week in April can still be really chilly.
I use Dunmore 16% for the most part. Gunieas get 20%. I also use the 20% on they quail and pheasants
I don't add oyster shell at all I let them have free choice. The guineas will leave it alone all winter but as soon as they start laying I have to refill it regularly.