9 Month old cronic hard stool

My 9 month old daughter has had tummy issues from birth. Pre-solid food she was only breastfeed. During that time her stool was always very runny, almost like water. It had no density to it at all. It was the yellow mustard color that breastfeed baby's normally have. Immediately after introducing solids she became constipated.She went 8 days with out a bowl movement. I made sure to feed her pears, prunes, or peaches everyday, along with an Avocado, which I have heard is very high in fiber. At seven months she began having a bowl movement everyday but with a lot of strain, and usually with help from me. We have been to the pediatrician repeatedly for this issue.She has been put on Karo syrup, olive oil, and finally stool softeners. The stool softeners did the trick after the 6th day of taking them, but as soon as she stopped taking them her poo became just as hard. Sometimes her whole bowl movement is hard sometimes it is really hard and dark at the beginning and gets a little softer and greenish toward the end, and sometimes it looks like little balls of very hard poop. While I breastfeed her I have resorted to supplementing her, between feedings, with water and a bit of pear juice. I was thinking she might not be getting enough liquid. But this also does not seem to be helping. Another issue that I believe is a result of her tummy issues is her weight. She was born a healthy 7 pounds 13 ounces and at 9 months only weights 13 pounds 7 ounces. She is constantly hungry, she breastfeeds 6 or 7 times a day, eats three meals a day, and snacks on avocados and mum mums. My doctor thinks she is just a little girl, but I think she may not be absorbing all her nutrients. She is a very fussy baby, and I believe this symptom might be related to her tummy issues as well. I just want my little girl to get this worked out!