Are you using a cow's milk-based formula? My dd struggled with constipation for the better part of the first year of her life until I stopped giving her cow's milk-based formula. She was almost exclusively bf except for formula in cereal and one bottle/night and that small amount constipated her. There are some great soy formulas on the market, that you may want to try.
Anyway, I ended up giving my dd flax seed oil, fish oil, and eventually laxatives every day until I finally saw a holistic doctor who told me about the cow's milk. Haven't had a problem since. Good luck.