Charlye has won the Easter Umbrella June Raffle! Next up: Dino Head.
So, as we all know, the cows in Cubic are kinda broken. No "milk" button appears when you walk up to them, except on rare occasions. However, there is a solution to this.
Simply trap your cow in a 1 block box, so it has no room to roam. Then, stand in the box next to the cow for 5 minutes. If you have an empty milk jug in your inventory, it will be filled with milk after 5 minutes of standing there. I'm not sure how long it takes for the cows to be ready to be milked again, I'll have to do more testing.
The box you make can literally be as simple as this: