Keep in mind that understanding and hearing are different things. It is possible to hear something and not understand it. This problem may be caused by high-frequency hearing loss.
Most consonant sounds are high in pitch and bring clarity to speech. They help you discriminate between different words, such as pick, tick, brick, lick, and sick. If you have high-frequency hearing loss, you may miss the consonant clarity sounds but still hear the volume from the low pitches.