Welcome Lann,
This can happen if the speech was only perceived as containing noises, or perceived as only containing rejected speech (that is, it is Rejecto working as designed). You can ask for all null hypotheses to also be returned by setting PocketsphinxController’s returnNullHypotheses to TRUE (you will receive hypotheses that are NULL and ones that are this string:@””), and you can ask for rejected speech to be returned in a hypothesis in the form of Rejecto-formatted phonemes by setting Rejecto’s deliverRejectedSpeechInHypotheses to TRUE.