Help! My goats never leave the barn anymore.

It is now summer of year 2, and I have 6 happy goats that produce milk. The only problem is that they never leave the barn although there is plenty grass growing right outside. I'm still pretty new to this game, so I don't know if I'm missing something or if this is some kind of glitch. Please help if you can. I apologize if there is already a thread about this; I looked but could not find it.

 

Questions. (apologies if they seem stupid, but I have to establish a baseline.)

1. Is the big door still open?
2. Is something growing or placed in front of the big door, like a tree, chest, machine or weed?
3. Are you playing with any mods?
4. Is it raining ?
5. How many days have they been inside now in a row?