You will be shown an excerpt from a computer science scientific research paper, followed by a question about the paper. Please answer the question. Do not include any text in your response other than the answer.

Context: Our beam search technique includes a length normalization procedure to deal efficiently with the problem of comparing hypotheses of different lengths during decoding, and a coverage penalty to encourage the model to translate all of the provided input.

Question: Why do NMT systems sometimes produce output sentences that do not translate all parts of the input sentence?
Authors implemented a coverage penalty to encourage the model to translate all of the provided input, however, it's not clear why sometimes NMT systems fail to translate all parts of the input.