Pals need to not be hungry or stressed to be assigned.
Put a food basket inside the pen to keep them from leaving to eat and getting distracted. Don't use raw/cooked berries in general, they don't provide enough food to meet the need within the 3 item per feeding limit. If you only use berries, their time spent working(e.g. breeding) will be shortened before having to go eat again. It is best to manually feed them to full if they are not full already.
Try to keep any other job stations away from the breeding pen to avoid distractions.
Don't both trying to block the entrance, they will just glitch out of the enclosure.
You can have some of the back outside the base circle, I would keep the backside of the hay inside the circle.