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Hello and thank you for requesting me. Sometimes dogs can slip tie which is why your other dog may have gotten pregnant. If you don't actually see them tie then you have to assume that they did not and she is not pregnant. This does not mean that there is something wrong with her just that the male did not make the connection even for a slip tie.
You can speak to the breeder if you are not satisfied she may give you another pup but the breeder will probably tell you what I did above though she/he may be able to offer the name of a very fertile male that is known for doing well even for a slip tie. They may also want you to prove through expensive testing that the female is not fertile. You would need to look at your contract with the breeder and see what they put in place for this especially if you made it known that you were getting her for breeding purposes. They may have a clause in there for return or getting another pup.
However even though your girl has gone through two heat cycles it is not recommended that you breed until the female reaches at least two years of age when all health certifications can be done on her and she is finished growing. The only explanation is that they did not actually tie. Reasons for this could be that the female intimidates the male so that he does not try as hard as he might.
Here are causes/ reasons /symptoms for infertility http://www.petmd.com/dog/conditions/reproductive/c_dg_infertility_female#.Ulxy6NKsiSo