A common cause of chickens not laying its the molt. At that age where you are this is not likely at all. But with chickens anything is possible. Stress may have kicked off a molt. But unless you see lots of feathers floating around, no. i'm mentioning this mainly to get it out of the way.
A common reason for chickens to not be laying is that they really are, but they are hiding a nest. Aart covered that. Butt check is also a good idea. That is a lot more accurate than any behaviors like squatting or appearance like bright red combs.
Is something getting the eggs? Many critters eating eggs leave evidence, shells or a wet spot. It's possible the chickens would clean up egg shells but they don't always. The critters that typically take eggs whole and leave no trace are snakes, canines, and humans.
I've had a problem with snakes, I don't think it is your problem. Snakes eat a few eggs (how many depends on size), go away for a few days to digest them before they come back for more. With snakes it's every three or four days you have a deficit, not consistently every day.
If it were a fox or coyote, you'd probably be missing chickens, not eggs. But sometimes a dog will eat eggs and not harm chickens. Does a dog have access?
I know it is creepy but I'll lave the thought of humans up to you.
So, what can you do? If they were ranging a common method is to confine them to the coop or coop/run to see what happens. Sounds like this is not a god option for you. Another thing you can do is to take an egg or two and put a mark on it, then leave it in a nest in the coop. If it disappears something is taking them.
I've had pullets not lay until they were 9 months old and they started the first week of December, the shortest days or the year. I do not use artificial lights to help them lay. Those three broke all the rules which shows there are no rules, just tendencies. I understand your frustration. Those pullets were how I introduced green and blue eggs to my egg basket. Their daughters typically started laying at 5 to 6 months like my other chickens so it doesn't even sound genetic. I have no idea what happened.
With Orpington I would expect them to start laying earlier. But you only have two. You have to have enough for averages to mean much and you don't. You may have just gotten unlucky and have two late starters. Did you get them from a breeder as opposed to a hatchery? Hatchery birds tend to start laying early. Unless the breeder specifically breeds for early laying, breeder birds can start later.
I wish you luck.