Speaking of getting it right or wrong. A lot of people don't concern themselves with this but I never feed layer feed unless the entire flock is laying. As you've found, one can't predict exactly when they'll commence unless on a strict lighting program and excessive calcium for an extended period with non-layers can damage the kidneys. For a mixed flock I make sure there's some oyster shell in a separate container and never let them run out.
Thanks, the breeder told me to put them on to layers pellets at around 18 weeks. As it was I waited till the bag ran out at about 20 weeks then did swap them over. I always have oyster shell available.
That's a shame. Have you noticed the combs and wattles getting redder/larger?
I assume you haven't added light.
So now they've been on layer for 13 weeks with no eggs.
If they appear healthy otherwise, I'd switch back to a higher protein grower feed around 18% till you get the first egg. Limit scraps and scratch to 5% of the total intake and you'll be sure they are getting proper nutrition.
If you add about 4 hours of light that should kick start them in a couple weeks or you can just wait it out.
At any rate, if they take much longer, 4% calcium is way too much since they have yet to build an egg shell.
Most people don't believe that excess calcium has an impact on health but the fact is that birds with visceral gout and urolithiasis show no symptoms until 24 hours prior to death. Then when a bird dies with no symptoms they don't get a necropsy and toss it up to 'just one of those things'.