Cold and wet is a bad combination. I would guess your calves are fighting a pneumonia that you might be missing until it causes them to get scours. Fighting 2 things will kill them.
Start taking temps, a thermometer will become your best friend. Treat anything over 102, no matter how young. Micotil, Nuflor or another drug for pneumonia should work.
Provide the calves with a dry place to sleep, a low board over a barn door works. Or move the calf shelter to fresh ground frequently.
Get the cows onto a better mineral program, and possibly better nutrition. Before the rest calve if possible. Too much milk never killed a calf, but too little then too much sure has.
If your way into calving, give the new calves vit.A, selenium (if you are in an area possibly deficient), and although I rarely condone mass antibiotics, 2 cc of penicillin at birth.
Jason Trowbridge Southern Angus Farms Alberta Canada