Your experience is very common. In fact, this is more likely to happen than not happen. When joints are immobilized (can't move) in a cast, the muscle fibers start to shorten. Injury to the bone and surrounding tissue may change the way the soft tissues work and move. This can also delay return to normal motion.
Loss of dorsiflexion (moving the toes up toward the face) is called a plantar flexion contracture. Three out of every four people have this type of contracture when the cast comes off. In fact, 22 percent of those people still have a contracture two years later.
Physical therapists are working to find the best way to treat this problem. Right now it looks like exercise is enough. Adding stretching exercises doesn't appear to help.