You're just unlucky. The parents have no effect on abilities, unless you're breeding for Dream World abilities. And Rock Head on Aggron isn't a DW ability. It's a 50% chance to get Rock Head, so you'll eventually get one.
And if you want to hatch Aron faster, you can use:
- Magma Armor / Flame Body
- Pass Power
- If this is on Black and White 2, Join Avenue's Nursery.