DD has had trouble with constipation every sinc...

DD has had trouble with constipation every since i started her on formula at 7 months. When she was on formula 30ml prune juice with her bottle once a day worked fine. Then i started her on cows milk, and i had to increase the prune juice to twice a day, and that still isn't really fixing the problem. The poor little one cries every time she poos, it is rock hard. She still poos at least once a day, if not twice, so she isn't backed up or anything. Its just super hard and hurts to push it out. Her regular diet it: 1 weetbix for breakfast 150ml Fruit and 1/2 slice of bread for lunch 150ml vegies and potato +/- pasta for dinner (she refuses to eat meat no matter how i serve it, and has a sensitivity to eggs) She drinks 3 x 150ml bottles of milk and 50-100ml water a day - cant get her to drink more... So anyway, i was looking it up online and i found that there have been studies that have shown that lactose intolerance can cause constipation. See one article here if your interested: https://wholewoman.com/forum/node/2906 My question is, have any of you had any experience with this or had the same problem with your LO?? I was thinking of switching her to Soy milk, which cant hurt if she will drink it, but i am still kinda unsure about the whole thing....