Dad's hearing aids are not really helping him. We've tried two different brands and providers, but he is still almost deaf much of the time, despite ear cleanings and my attention to cleaning the hearing aids. Dad's understanding of what he does hear is certainly part of the problem, but I'd like to help him if I can. I see there are inexpensive OTC aids and wonder whether anyone has experience with these.
Write a simple sentence on paper and see if dad understands from reading what you’ve written. Have him read the sentence or tell you what it means.
If it’s comprehension see a neurologist or speech-language pathologist.
If it’s hearing, you already know about audiologist.
Of course, it could be both.
If your Dad has been going to an audiologist and being fitted for hearing aides, this is the person you should be talking to. Is he going back for the tweeks? His hearing gets worse, his aids need to be adjusted for that? Hearing aids are mostly for volume. My DH will tell you that how he hears is like static on a radio. Turning it up may help you hear the words better but there is still that static. He reads lips better than he hears.
According to your profile it's your mom that has dementia, but I'm guessing that perhaps your dad too has it because not being able to comprehend the spoken word is definitely a sign/symptom of it.
So perhaps it best that you just save his/your money on any more hearing aides and just let him make due with what he now has.