When mycelium shows in the valleys the room changes: temperature down towards the fruiting band, fresh air in, carbon dioxide down. ICAR-DMR reports pinheads 3 to 4 days after that change.
Timing is the risk. Change too early and the mycelium has not reached the surface, so pinning is thin and late. Change too late and it overgrows the casing into a dense mat that pins poorly. NHB, following ICAR-DMR, adds that temperature above 26 to 27 °C after casing must be avoided, so a warm week is a problem to fix.