Ask your veterinarian to examine the animal; or at least, take a fecal sample to your veterinarian for a parasite examination and fecal culture. Your veterinarian can determine if there are worms or coccidia from a fecal flotation. Worms have developed resistance to some dewormers and it may be necessary to use a different product. Some bacterial infections, such as Salmonella, may also cause chronic diarrhea. Fecal culture can help determine if this is the problem.
In some cases the diarrhea may reflect a digestive disturbance caused by the diet. It may be helpful to re-populate the rumen with microbes. You can buy a paste at a feed store or from the vet that contains rumen microbes. Follow label directions.
Then: back the calf down to a 25% grain and 75 % roughage diet. Leave the calf on this diet until it is rid loose stools. Thereafter, work the calf back up to whatever diet you want. If you are finishing the calf for slaughter, 80% concentrate:20% roughage would be my suggestion for a final finishing diet. If the corn is processed, make sure it is coarsely ground, meaning there are some whole kernels present. Grinding corn too fine will cause problems.