Baby Constipation? Advice to new mom?

I have a almost 3 month old, and i have had to change her formula 3 times already. The first time was Enfamil for spit up, second was constipation on the Similac sensitive, and now Similac Soy. The doctor tells me to keep using white grape juice twice a day and gave her suppositorys that i have had to use 4 pr 5 times because she hadnt had a bowel movement in 4 or 5 days. But they always worked, i just dont need to keep doing it and really want her to not be constipated and fussy so much and i wanna help her...does anyone have this problem?