The biggest contributing factor to reduced speech understanding with hearing aids is that the user waited too long before getting hearing aids. Long-term, untreated hearing loss causes reduced function of the auditory system. The network of neurons that exchange information back and forth between the ears to the brain, gets weaker and weaker with lack of use. The average person with hearing loss waits 7-10 years before taking action to get hearing aids. If you were to reduce your motor function by sitting in a wheelchair for 7-10 years and then were expected to be able to stand up, run, jump, and pivot with excellent coordination, you would not be physically able to, due to the atrophy or decline of not only the muscle strength, but the communication capacity between your legs and your brain.
A second reason why words may still sound unclear even with hearing aid use is that the user has significant damage to the tiny hairs in the cochlea. Certain hairs are responsible for disguising two frequencies that are close together. If that distinction cannot be made by the brain, then two sounds, such as an f and an s, are unclear and sound very similar.
An audiologist can take measures during a hearing assessment to determine how much auditory weakness is present and can give you an expected degree of success. However, the best determining factor for how clear sounds will be, and how well one will understand speech and conversation once fitted with hearing aids, is how soon the hearing loss is treated.
The recommendation is to place hearing aids on mild hearing losses, and not to wait until the loss is moderate, severe or profound. The outcome just likely won’t be the same. Remember, successfully wearing hearing aids is time dependent.