Very nice coop and run.
I think if you wanted to speed things up, I would gently pick one up and put them in the sand.
I never had trouble with my first chickens coming out of the coop into the run. I had them in the coop for 3 wks until I finished the run so that may have something to do with it.
My silly ducks on the other hand. We got lots of rain here in the Pacific NW a couple weekends ago and I had mud were I had grass.
So I dragged two 3/4" rubber horse stall mats into their run. They were so affraid of the big black mats that they would fly over them to their night shelter and back. Wouldn't step foot on them. Wouldn't touch their food or water that were sitting near them.
I had to move the food and water. They wouldn't even go in their little pool with fresh water. At bedtime, I had to chase them down with the broom to get them to go in their shelter.
I sprinkled wood chips over the stall mats and it took them another 4 days to walk across them.
My goats don't like anything new either. I moved them from sleeping in the chicken run to their own 12 x 12 shelter and they cried their heads off for a good week.