My girl rarely has constipation. I have switched her formula milk a few times since newborn until now (she's almost 14 months old). She had Mamil, Similac, S-26, Enfalac and then back to Similac (Gain IQ).
What I have avoided giving my girl since she started her semi-solid food is cereal and cornflakes as these dried snacks do cause heatiness if the toddler started to produce hard stool. What I have NOT give up is give her mashed/puree pumpkin, sweet potatoes (which is very regular in her diet), rare tomatoes (she self-feed) and Vitagen/Yakault (given to her when she didn't poop for a day). She is always happy to drink plain water which I give her daily/regularly.
Try adjusting her diet for a week such as cut down on biscuits and no cornflakes to see if this will improve her bowel movements?