I saw this just last year in a litter of kits, it was the biggest kit that first took it first and pulled itself along like a seal pup, he lost this kit, however he gave the rest yoghart made from goats milk, ( got from asda ) and the other two made 100% recovery, it was the first time that I had ever witnessed this in 35 years of ferrets, these kits had been reared on woodpigeon carcasses perhaps that had something to do with it ?, but I think Malt is on the right path with diagnosis of Calcium deficiency, please keep us informed and I hope all is well.