20 weeks is early to average, not all birds start at the same time.
Do they free range?
Are you sure they all are laying?
Signs of onset of lay---I've found the pelvic points to be the most accurate.
If you touch their back they will hunker down on the ground, then shake their tail feathers when they get back up.
This shows they are sexually mature and egg laying is close at hand.
Combs and Wattles:
Plump, shiny red - usually means laying.
Shriveled, dryish looking and pale - usually means not laying.
Tho I have found that the combs and wattles can look full and red one minute then pale back out the next due to exertion or excitement, can drive ya nuts when waiting for a pullet to lay!
Dry, tight, and smaller - usually not laying.
Moist, wide, and larger - usually laying
Pelvic Points 2 bony points(pelvic bones) on either side of vent:
Less than 2 fingertip widths apart usually means not laying.
More than 2 fingertip widths apart usually means laying.