I was looking at your photo, and noticed the only one that is laying is the only one with a bright red comb and waddle. My girls are only 10 weeks old, so I have quite a while more to wait, but I have read on this site alot and also my neighbor who has a huge flock of Buff Orpingtons that their combs get very red when ready to lay. Your other three don't look as mature as the Barred Rock.. She also told me her buffs usually start to lay 23-24 weeks.
So, I am sure you are doing everything fine, add the oyster shell, and just be a bit more patient. THey are on their way!
Good Luck, and let us all know!
MB