Is it possible that an orthopedic cast fails to keep a bone straight?

It is. Though more often then not, it should. If you had a lot of swelling that reduced then the cast possibly may not be snug and technically, movement can occur. Possible? Yes. Likely, probably not.
They may. When an orthopedic surgeon is presented with a fracture he relies on his training and experience to decide the best way to treat it. Some r borderline, casting or surgery and internal fixation. If a cast is used in this type there maybe failure; then you become a surgical candidate l.