I am having trouble IV breeding on pokemon white please hepl?

ok I have been trying to IV a pokemon with 31 ivs in special attack,and I did the breeding Calculator and the chances are 1 in 70,and I have breed that amount of pokemon and still no pokemon with 31 ivs in Special Attack but one of the parent pokemon had a 31 iv stat in HP so I have bred a lot of Pokemon with 31 ivs in HP,please help what do I do