What will help with baby’s constipation when probiotic packets arent working !?

ash

Changed her to Gerber soothe formula which I was told was specific to babies with constipation issues also started giving her probiotic packets once a day in her milk which people said would work wonders and nothing has changed! Prune juice also has done nothing to help! Her stool is the size of what should come from a toddler & shes 3 months old, please share if you have any tips!