Retailers who keep the same % markup are making lots more money.
Assume 50% markup.
Retailer buys lumber for $100 sells for $150. Makes $50.
Retailer buys lumber for $200 sells for $300. Makes $100.
Retailers always keep their percent markup the same and make much more money. But claim they don’t since their % markup is the same.
Or alternately, at $150 retailer sells three times as much as at $300, so he makes less money at the higher price. There's no question that people are cutting back and waiting for lower prices.
Or another alternative: retailer can't get any material from the mill. He still has to pay all of his expenses even though he has nothing to sell.