formula & constipation

So I've strictly ebf my daughter until 6 months old. Then I was pumping during the day & breastfeeding at night. Then about 6.5/7 months I would add a maybe 2 formula bottles during the day on top of breast milk & breastfeeding. She's 9 months old & STILL gets constipated if she has more than 2 bottles of formula a day. (I'm trying to work on being done with breastfeeding which is why I'm introduced the formula but just didn't think formula would still be a constipation problem going on almost 3 months of doing it. That's the only reason I haven't fully switched, she uses Enfamil gentlease if that matters any!