Deafness in the elderly.
Mum is 84; no dementia but has been very hard of hearing for a number of years and at first she didn't tell anyone she had difficulty in hearing. My ex and I simply thought she was a combination of daft and stupid, because we got fed up with having to keep on repeating things and speaking LOUDLY so's Mum could hear what we were trying to say to her. Even then conversations were difficult because she would often break away in the middle of a conversation and go do something else, which my ex and I found incredibly frustrating.
Eventually after one visit, Mum finally told me she'd had less than 50% hearing in both ears for a number of years - but I do wish she'd told me at the start so that I would have understood. Mum does have hearing aids but never wears them at home because she doesn't want to waste the batteries, making telephone conversations next to impossible because she rarely hears the phone ring, even with an amplifier, and when she does pick up the phone the TV is on in the background so loud I can't hear what she's saying!
Face-to-face, Mum tells me that to make it easier for us to talk to her & for her to follow a conversation we need to look at her full-face on so that she can follow what we're saying by lip-reading (which is how she managed for years before letting on to us she was partially deaf). Speak slowly and clearly, enunciate precisely, open your mouth properly when you talk to her, and don't mumble when you speak. If she's not looking at you when you want to say something to her, I have to attract her attention by tapping her on the shoulder, so she will then turn her face to look at me full-face on.