The brain works hard
Many people find it challenging to follow conversations in some places, like in a noisy restaurant. This is because speech is made up of a large number of different sounds, put together in very rapid flow. Our brain constantly prioritizes and organizes all these sounds for us.
When it comes to hearing, it may come as a surprise to learn that the brain works harder than the ears. This is why in noisy environments, such as in a crowded restaurant, it can be very frustrating just trying to follow conversation. Even people with no hearing loss can find this challenging. Ordinarily your brain will be able to sort through all information you apply your attention to through a cognitive process: Put simply, the brain organizes the sound environment, selects the desired source and follows it. For people with hearing loss, however, the brain has to work much harder to make sense of sound because the input it receives from the ears is softer, less detailed, and/or unclear.
Some sounds are heard better than others
The high-pitched consonant sounds like f, s and t are easily drowned out by louder, low pitched vowel sounds like a, o and u. This results in a person with hearing loss complaining that they can hear that others are talking, but not what they are saying.