Why do chickens abandon their nest box

[breezy]

I havent had any eggs for a week. I thought they had stopped laying due to the heat and the beginning of an early molt. Today I found 12 eggs under the coop. Based on the color size and shapes its clear all of them have decided to quit using their nest boxes in the coop. I had changed the bedding from straw to pine shavings, would that make them quit? I changed the bedding back to the old stuff and put 2 eggs in the nest and left them there. Tomorrow I plan on putting a skirting around the base of the coop to stop them from getting under there. I have 4 hens and 2 nest boxes in the coop.
What would cause them to stop using the coop nests and what else can I do to get them back into their nest boxes?

 

[mstricer]

Mine do that if someone else is on one of their favorite boxes and once one sees her laying in the garage some tend to be followers. I collect eggs from about 4 different spots. You have enough nesting boxes. I use both straw and pine shavings they tend to like the straw.

 

[midget_farms]

may be cooler under the coop than in it. I don't thing chickens care what type of shavings are used.