Birthing too long, Oxygen deprived. Did you try a shot of dophram or epinephrine? Dophram will help to stimulate them to breathe along with A/R. Epinephrine is good at jump starting and extremely weak calf having troubles getting that first breath.
One way to tell if it was birthing to long...it's the not the only way to tell, but one way, is if the calf is born yellow, the fluid around him is yellow.
A weak born calf can also be the result of either poor nutrition or poor mineral consumption. Hay is only as good as the weather it was grown in. The results of heavy rains or drought in the growing season decreases the protien of the hay. So you might be feeding them alot, but if the protien and tdn are not there you will get weak calves and poor quality milk and colostrum.
Mineral is very important as well to the birthing of healthy calves
Exercise for the cows is very important to keep their muscles strong for calving. An overly fat cow will have troubles giving birth
Next, BVD. It is a serious disease and calves born with it can be born weak and then die.
Good luck with the rest of you calving. If you have another calf born like this or die quickly, get it posted