How come you dont interceed before the forth day if you noticed them abandoned and weak on day 3...maybe a more active intervention should be done in the future just till you can isolate the issue if possible. When our heifers drop their calves we go up as soon as possible to see the sex of the calf...one good rule I found works well, unfortunately too late for one of my calves is...if they arent up and running within a couple of hours, you should watch them carefully. With 4 of our calves if we didnt get to them within the first day to check their sex....it was good luck to us..couldnt catch them...however with the last one we were able to walk up to it for 2 days straight, it hardly moved from the same spot...I immediatly took it to the vet, we bottle fed it for a week at which time it was determined he was blind in both eyes and had brain damage, (thus the inability to walk well and carry his head) and was put down. Yeah it cost money for those who think I should have just let it die, but then I wouldnt have learned a darn thing would ? And one more thing I learned is when something is wrong they go down hill really FAST. The best for you in the future and I hope you dont have anymore losses.