I have a chihuahua pup that seems constipated...how can I help her?

I recently purchased two chihuahua puppies (14 weeks old) from a local breeder. I asked the breeder what they were being fed and have stuck to the same brand, Royal Canin....however one of the puppies has yet to "relieve" herself and seems constipated (2 days without a bowel), while her sister defecates 3-4 times a day. They both have no issues with urination and their actions seem normal and healthy out side of this one condition. Is there something that I can give her that will aid her in relieving herself? Please help out if you can!! Thanks in advance!