P.H.
I had the same problem with my son, who is now 14 months old. His pediatrician recommended putting a small amount of Karo Light Corn Syrup in his bottle, along with the formula. It works! And still does ~ I just had to put a little in his sippy cup last night to help get things "moving" again :) Just make sure to use the light syrup - should be clear or slightly yellow in color.
Good luck!