My first suggestion is to find a local cattle farmer who doesn't mind coming and giving you a bit of help and advice on rearing calves. It isn't as easy as it looks and you cant go wrong getting some assistance from a veteran.
What kind of grain are you feeding and are you giving them as much as they want? Too much grain can cause acidosis and bloat; the stomach blows up very round and hard and puts pressure on the diaphragm making it hard for the animal to breathe. In young stock this very often leads to death, but I have seen it in greedy adult cattle too. From your description, it sounds like that is the problem. Make sure you provide your calves with plenty of hay and fresh water, and keep the grain to a small amount per feed say .5kg (about 1 lb) two or three times per day so they cant gorge. You can provide ad lib grain to calves before they are weaned as it encourages the weaning process, but once that is all they are getting along with hay, you have to be careful to limit the quantity they have access to.
If you are concerned that one of you animals has had too much grain, dilute about 150- 200 g of bicarbonate of soda in water and tube them with it (you can just use an old wine bottle or the like; tilt their head up slightly and insert the bottle through the side of their mouth where they dont have any teeth and slowly pour it in, making sure they dont inhale any).
I really hope that helps; dont hesitate to put any other queries up on here, but the best advice I can give is to find yourself a pet farmer to help you out until you get the hang of it!