Cows are stuck / not milking
Cow AI currently bugs out if they are fully fenced/walled in. This can be solved by simply leaving an opening in your fence around the barns.
Upgrading to a tier 2 Barn can also sometimes bug out the herd, so sticking to tier 1 for now is safest.
Walling off cows completely can actually dramatically impact performance as their desperate AI seeks solutions.
We are working on a fix for this for an upcoming update.