Due to the power of DNA testing it is very rare for a paternity (or maternity) case to be inconclusive. However, very occasionally we may detect a natural change in one of the DNA markers which means that it does not match as expected.  This complicates the analysis but generally, because we test so many markers, it does not significantly impact the statistical result in cases when both parents are tested.  However, if only one parent is tested, this can lower the statistical results so that we are only able to confirm that the two individuals are closely related, rather than conclusively confirming the nature of that relationship.