I have mix fed, FF and BF DSs 1-3 and IME kids are all different and do differently well with different milks.
Is she keen on solids? If so, make them more liquid, ie add more formula to porridge, fruit puree etc. Mine were wild on smooth puree made from (raw) apple, pear, plums, kiwi (we had no sensitivity probs with kiwi). Oh forgot, pineapple, also brill to produce soft poos.
DS1, now almost 6, has had lose poos all his life, DS2, almost 5, has been Mr Constipated from the word go. Some of it is to do with slower gut transit times, so is constitutional in the presence of same diet.
It does sound though like she is not taking lots of fluids. Try yoghurts, jellies etc? How about blended lentil/bean/pea soup??
(You can always try watering mild feeds down a bit, don't tell anybody I said that).Yes, and agree with bubblymummy, try easy digestible FF, there are lots out there. Try prune juice: only needs small amounts, kids tend to like the taste, not too medical.
If all else fails and because a bowel can get used to being sluggish, go see your GP. Any changes you make/ medication (ie Lactulose, senna, Movicol) you use, you need to persevere with for a longish period of time. Do not just stop because there has been the odd soft motion, keep going to "reeducate" her bowel. I know I will get shot down in flames, but if she is sore and cries when passing motion, consider paediatric glycerin suppositories: they work, kids no not mind, it is NOT CHILDABUSE, and perfectly natural way of getting rid of "poo plug" allowing softer bowel content to follow and giving her relief.
Good luck! It is awful to see them in this much discomfort, isn't it? DS2 now ask for pear himself when he feels things are getting a bit uncomfortable...