Aggron has a low Special Defense stat. Only base 60. That is low.
To beef it up, train it more in SDef. How? Feed it Zinc, which raises SDef by a pinch.
Also, you can Surf the waters of Hoenn and kill Tentacool, which give off Special Defense EVs. If you kill 252 Tentacool, you will have a pretty high Special Defense stat (for an Aggron).