Are there times where you can't get 100% correction of club feet, why?

Yes. Most club feet are now managed non operatively with casting as long as they are addressed as infants. 100% correction is not always achieved as it is a genetic deformity. Pediatric orthopedists are the experts in this topic and i would defer to them for further advice.
Yes. Scarring of tendons, ligaments, joints, or changes in the bone anatomy can sometimes cause corrections to be less than 100%.