I tried to breed my Contary Serperior with a Ditto but the Snivy breed only had Overgrow! Will that be the result every time or is there a chance it will have Contrary? Also, code for Contrary Serperior here: http://www.pokemon.com/us/pokemon-news/three-stars-of-unova/
When Breeding Hidden Abilities on Pokemon the gender plays a role. Males have a 20% chance to pass it down While Females have 60%.
Your serperior is probably male so it has a small chance to pass it down but not very likely.