Ok so let's go through the best way to do this - If you get a Ditto with the nature you want on your baby Tyrunt, you can breed this with the Tyrantrum and the baby will have a chance of having the HA sturdy. Also, if the Ditto has an Everstone, the baby is guaranteed to have the same nature as it. The HA has a chance to be passed down whatever the gender of the parent (60% for a Female Tyrantrum and 40% for Male). Otherwise, you're all set. There is always a chance for the HA when breeding with Ditto. But if the TTrum is Male, the chances are lower than a female. Or you could GTS I suppose. Your doing everything correct, you're just unlucky XD