Have you confined them for at least week or more?
You can't just 'keep a close eye' on them......
.....unless you literally did not take your eye off them for more than 5 minutes all.day.long for 3 day running.(which is impossible IMO)
Have you checked vents and especially pelvic points?
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!
Vent Appearance:
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.