What Causes Difficulty In Passing Stool After Taking Dexolac?

my baby age of 2 months 8 days previously we used feed Dexolac special care as weight of the baby was less than 2 kg. Now, she has gained weight. Present weight is 3.4 kg. Doctor has stopped Dexolac special care and started feeding Dexolac 1.After feeding Dexolac 1 she is not doing stool for 2 days and we need to give Neopeptine. Stool is very hard which was not there when she was taking Dexolac special care.Now doctor has advised us to go back to Dexolac Special care. We are confused.