Why aren't my turtles breeding?
Your farm needs to be on the "home beach" (the beach on which they spawned/hatched) of the turtles you breed. So you either have to base your farm around the turtles or find a couple turtle eggs, pick them up with silk touch and bring the to your farm, and wait for them to hatch.
They take a minute to actually lay the eggs after breeding, it could also be because you are not on their home beach, in which case find turtles that are based on a beach near you