Chickens can't go into coop

I'm not using any mods to the game. I am certian there are no obstructions. I even expanded the area, and added a second coop to see if they could go in that one. No luck. I bulldozed the lot, finally, and put a prebuilt house in, and tried chickens again, still nothing, but when I bulldozed the terrain, it finally worked. My terrain was flat to begin with, and I had even tried using the flatten terrian tool, but since it was already flat to begin with it didn't do anything. It finally worked after I bulldozed everything and started fresh, but changing the terrian. I think it had something to do with the fact that I changed grass to dirt in the animal pen.