My daughter (now 9 mths) has been going through the same thing off and on since she was about 6 months old (when I stopped nursing and switched to Soy formula) - and I recently figured out why - I was mixing her bottles of formula too concentrated.
That is, I would put the 4 scoops of formula powder in the bottle (what the can says you need to make 8 oz) then add all the water it would hold - thereby (I thought) making 8 oz of formula. But that's not right! In order to make 8 oz of formula (or any amount) you have to start with that much water first and then add the formula to it - which ends up being a volume greater than just the 8 oz or whatever, but is a correct mixture. By not adding enough water, she was getting just a bit too much formula to water and was constipated all the time! We've adjusted our methods in the last 3 weeks and it has made ALL the difference!
If this doesn't help - Dark Karo Syrup in her bottles (my Ped told me to do this) - about a tsp per bottle - also helped her.
Good Luck!