The position that a calf eats is VERY important! The neck must dip down, and the nipple must be lower than their backbone.
Think about the height of momma cow’s nipples. They are low. The calf must dip its head low and reach under her to feed. This is important to simulate when bottle feeding to prevent serious problems.
A calf must suck to get milk. The sucking come from the lungs creation a vacuum to pull the milk in. The anatomy of the calf is such that if the head is upright, above the backbone, the act of sucking will allow some of the milk to reach the calve’s lungs and be inhaled! This can have devastating, even lethal consequences. Milk in the lungs is asking for an infection or even pneumonia.
Sometimes a new calf rejects the bottle at this position. If they didn’t nurse long enough on mom to establish the pattern and position, the calf stops sucking when the bottle is dropped low enough. In these cases it is important to train the calf properly. Keep lowering the nipple just a tad lower where they will suck but just barely. It helps to hold the bottle more horizontal until they get the idea. Just do not let the calf get milk when their head is above their backbone. after a feeding ot two they will get the idea.
Remember, calves need to eat low and slow. (slow is addressed in a different post).