"Though inter-breeding was not utterly disastrous, the mixed offspring of modern humans and Neanderthals did not, in general, fare well. Genetic tests show that most Neanderthal DNA was rapidly lost from the modern human genome. One theory is that mixed children grew up to be less fertile, or were less likely to reach fertile age, meaning their DNA vanished quickly from the gene pool. "

Or RH- blood, which makes it difficult for the mother to carry the child...

Who do you work for?