It could be that your hearing isn't good when you are around a lot of noise too. Some work places are noisey. Also, some people talk in tones a person may not hear well at too. Seeing this happens to you more than you would like, it's best to get your hearing checked so you can learn what your actual hearing range is. This way you can inform others that you can genuinely not hear them in certain situations and to please don't think you are choosing to ignore them.