I am not a speach therapist, but the following are some ideas.

Some people cannot hear certain sounds and so of course then they cannot say them properly. If a person has tone deafness then that may be used a symptom in some cases.

Others have not learnt to say certain sounds when they were young and when older may not be able to grasp or have the ability to learn them especially if it is a forgein langusge.

Example: I know a person that has no difficulty with any English sound of any letter. However when he was learning Japanese, each consonant had a different sound according to the vowell used with it. So of the four vowells used with the 26 consonants there were 104 different sounds required. He just could not say one of them. He could only master 103 sounds in Japanese. He has no speach problem to be treated.
Another case is of a grandfather that cannot say th. It comes out as tf. Yet his grandaughter says th clearly and even says th instead of s. So a horse becomes a horth.
Another man that also has to do public speaking comes from Newfoundland and he cannot say the letter h. I think this is common there.

Buy the way why are Newfy jokes so simple? So the rest of the world can understand them.