Sometimes blood-poisoning (septicemia) causes the problem you describe. The medication controls the infection, but as soon as the medication is gone the bacteria re-seed from the site of infection and make the calf sick again. This situation usually occurs when the navel is infected and abscessed, but the infection can come from a variety of places including the lungs, portions of the intestinal tract, or joints. In this case treatment may involve choosing a more appropriate antibiotic regimen that eliminates the infection.
Sometimes repeated bouts of scours occur because the gut has overgrown with yeast because of using antibiotics which have wiped out the good intestinal bacteria.
The trick is knowing whether antibiotics are necessary (as in the first case) or causing the harm (as in the second case). Because the calf will eventually face the second problem, consider stop administering the pills and treat only for dehydration by giving supplemental fluids. It would be useful to monitor the calf's behavior and appetite and check for fever regularly for a few days. If a veterinarian could examine the calf a better diagnosis to the problem is possible and a suggest treatment could be imposed.
