Many different factors come into play when we are listening and trying to understand. Some of these are obvious - hearing the conversation, lip-reading, and body language.
Others are more subtle such as the differences in loudness of sounds (dynamic range), the signal to noise ratio, reverberation, distance from the source, or other temporal (timing) disruptions.
The first two of these other elements can contribute positively to give you additional cues to help you process the sounds you hear. The others lessen your ability to decode the signal and extract the necessary understanding.
One thing that hearing aids all use is compression. This is where softer sounds are more amplified than louder sounds. This does have some advantage, but it can also work against you. An example of this is listening to television while wearing a hearing aid.
TV audio is 'companded' where the softer sounds are emphasised (made louder), but the louder sounds are not. This 'distorted' sound signal (low dynamic range) is not clear for someone with a hearing loss and the usual answer is to increase the TV volume.
This louder sound from the TV is then less amplified by the hearing aid (or compressed) which reduces the dynamic range even further and removes more of the vital clues you need for understanding.