Inadequate animal training. Each and every animal must learn that the fence hurts. Build a handy training fence, preferably on heavy, wet soil. Flag the fence for visibility and force the animal to try to cross the fence. Wires tied tight to each fence post.
Sunlight deteriorates plastic.
Usually black ones are treated to resist degradation by ultraviolet light. Poor quality insulators usually turn white or clear after a few years in direct sunlight. Staples driven in all the way. When using plastic tubing as an insulator, don t staple it too tight. You may end up spending a lot of time looking for a short, when it really is a result of connecting a wire to a ground wire.