Since you said you find them outside their run I'm assuming you let them free range? If you're letting them out early in the day, try keeping them in until late afternoon, like 3-4 PM and see if they will use the boxes.
I have a flock of 20 that free range but I've only found one egg outside and that was more than 2 weeks ago. I see my hens head for the coop if they need to lay. I leave the coop door open for them when they are free ranging. I also don't let them out until late afternoon because the majority of eggs are laid from about 8 AM to 3PM.
Also, if you can catch one trying to lay that is not a nest box, put her in there. I only had to do that one time with my first layer in the flock and the rest followed her lead.