Prune juice not working for babies constipation

Help! My 11 month old is severely constipated. I have given her 4 oz of prune juice in the last 24 hr mixed with water and formula,1oz each time bt it didn’t work,I have tried avocado,mango pro biotic yogurt bt nothing seems to work.now it’s really making me worried cause prune juice was my last hope , It worked for everyone why is it not working for my baby?what am I doing wrong?should I give her 2-3 oz at one time instead of 1oz each time?